Steve another place to look for a missing seal is where the heater core in and outlet come through the firewall,there is a foam seal that seals between the two copper pipes, if its not there engine heat can get into box through there, I didn't have to take anything apart, just the floor shield, just shoved it in tbe best I could, Dr. Rebuild has em, might help.

You said you put in a hot water shutoff valve, that fixed the heat problem in Ian, but hasn't in yours, so try looking under it like all the other suggestions. Heat is leaking in from the engine somewhere for sure. Good luck and let us know what you find that stops it, since almost all vettes have some heat coming in at some time.

I have improved the heat situation.  No single improvement has made a major difference but each has helped a little.  We are making progress!!

What I've done so far-

• Sealed the inside passenger plenum with windshield installation sealant.  I could not reach the entire inside of the plenum but I think I felt some gaps that are now sealed.
• Took apart the passenger side map pocket to expose the duct work.  Made sure all the vacuum doors were working properly.
• Adjusted the Bowden cable to be sure no air was leaking past it into the heater core chamber in the "cool" position.
• Sealed the seams of the heater box in the engine compartment with chimney and flashing sealant.  It is heat and moisture resistant and flexible.
• Replaced the A/C evaporator drain boot.  Mine was missing.  Without it, there is a 1" hole in the bottom of the heater box that will let in hot air.
• Checked the weather stripping on the hood for a good seal against the fcowl.  Found that it was tight on the ends but had a gap through the middle.  I will replace that soon.
• Installed a shut off valve in the heater hose from the motor to the heater core. 

All of these things have improved the heat coming through the duct work.  About a 40% improvement overall. 

In addtition, I have also made the following improvements that have also helped the heat in the cockpit-

• Removed the rear console, center console, and center instr. cluster and wrapped the transmission tunnel carefully with insulation from Home Depot.  The insulation is a sandwich construction of foil on the outsides and a plastic "bubble wrap" core on the inside.  It is a really great insulator and is not expensive.  I also extended the insulation under approx. half the floor carpet.  Sealed all the seams with a couple of layers of duct tape. 
• Removed and taped over the air ports over the drivers and passenger's feet.  They just don't seal well on my car.
• Replaced the lower shifter boot.  Mine was torn and had big gaps that allowed a ton of heat in.
• Installed the foam piece that sandwiches in between the bell housing and the firewall to prevent hot air from blowing through the transmission tunnel.
• Before I put the dash back together, I wrapped the duct work with the same insulation. 
• THIS IS IMPORTANT-the steering column sits right next to headers in the engine compartment and the heat transfers up the column into the cockpit.  After a good drive, reach up under the dash and feel the steering column. It is so hot you can hardly touch it.  I wrapped it with the same insulation and I could definitely feel less heat at my feet. 
• Also, regarding heat on the feet, I replaced the clutch rod boot iin the firewall.  Mine was cracked and had big gaps that let the engine heat in through the firewall.
• Checked all the grommets for a good seal around the accelerator cable, tach cable etc.

All of these things have helped and I would estimate the heat reduction in the cockpit from leaks and bad insulation to be reduced by 60%.  A good start compared to where I was a while ago.  Also, you don't need to be a certified mechanic to make these improvements.

Mickey

Hey all,

 I just installed a shutoff valve in my car and the difference in temperature is incredible...

 In case somene wants it the NAPA Part number is 660-1414....

 Best money I've spent so far on a long shot.....

 I also fought this for awhile. I did everything that you guys did and it got better. Then I bought the lizard skin ceramic insulationg system and what a difference. It was quite pricey but what a difference. I like it so well that I am stripping out the interior of my street rod to do it. Worth every nickle!

JimG, just to let you know--

I went to NAPA today and they had the exact part you listed. It looks like
an ideal solution--especially compared to that humongous Hummer-size
shut-off valve pictured in the article that's linked above (no disrespect to
the author, but man is that contraption big!)

You'll get a kick out of this--the guy behind the counter looked surprised
when I ordered the part by number rather than description. So much so,
in fact, that he asked me where I'd heard about it.

He seemed impressed when I told him that a fellow member of this forum
had steered me to it. I explained that our Vettes are nuclear-power-
plant-style generators of heat, and that this valve might just be the
answer our hot-footed crowd is looking for...

so thanks very much for the tip--and the precise part number!

[QUOTE=SFVette]JimG, just to let you know--

I went to NAPA today and they had the exact part you listed. It looks like
an ideal solution--especially compared to that humongous Hummer-size
shut-off valve pictured in the article that's linked above (no disrespect to
the author, but man is that contraption big!)

You'll get a kick out of this--the guy behind the counter looked surprised
when I ordered the part by number rather than description. So much so,
in fact, that he asked me where I'd heard about it.

He seemed impressed when I told him that a fellow member of this forum
had steered me to it. I explained that our Vettes are nuclear-power-
plant-style generators of heat, and that this valve might just be the
answer our hot-footed crowd is looking for...

so thanks very much for the tip--and the precise part number![/QUOTE]

 Glad to hear it.. Make sure you put it on the heater hose leading from the intake and get those hose clamps nice and tight...

 Hope it helps and this is what this place is all about !!

 Jim

"If your 70 is like my 68 the hot air from the floor vents are directly connected to the engine compartment which allows the hot engine air to enter very well. Good design for Alaska. My heater leaked and was disconnected & I had no air installed. It was like sitting in a Sauna. So I pluged the great vents on the floor..added air conditioning & installed 2   7' spal fans right  behind the frt tires. These fans pull alot of heat out of the engine. Took a ride last Sat 93-95 degrees I was cool from the waist down & the engine never went over 195 degrees. Had to turn on the 16" spal puller at stop lights. Alot of fans going but I'm cool.

 Almost forgot I install the aluminium heat sheild too under the carpet

Alan"

Hey Id like to hear a little more about your fan install if ya dont mind?

Glenn

Glenn,

I don't know what else to say except you'll have to use relays or install another circuit because the fans draw quite a bit of current. The 7" spal fans fit in a vet almost perfect . I did have to make a small sheet of sheet metal behind the front tires to keep the tires from kicking up thing into the fans. The 2 7" spals draw 1400cfm together & on a hot day the heat is so high you can't put your hand under the fans. I run the 7" spals all the time. Haven't needed the 16" to much since I moved the 17" flex alittle closer to the radiator. I have pictures I can e-mail you.

Alan

[QUOTE=stevep316]heres the link i tried to post-

http://temp.corvetteforum.net/c3/joeveto/Hot_Water_Shut-Off. shtml


see if that helps-
ive installed one on mine, and it has reduced the inside temp big time!
[/QUOTE]

I also installed a shutoff valve on the return line to the water pump.  Without it, heat travels up the hose and into the heater core, even without any flow.  Just another trick i learned on these forums.