it's not so much "have to" as that I'm growing weary with having to spend money fixing things like weatherstrip, windshield wipers, AC, electrical wiring, gauges, all the little leaks, instead of spending money making it go faster.  not to mention it costs me twice as much because I don't know how to do any of it myself.

SIDE PIPES ARE THE COOLEST LOOK. wHATEVER YOU DO , DO NOT PURCHASE YOUR HOOKER COMP SIDEPIPES WIHT CERAMIC COATING FROM SUMMIT RACING.i WENT THROUGH 3 SETS IN SIX MONTHS BECAUSE THEY RUSTED. vERY FRUSTRATING. i SPENT THE EXTRA $400.00 FOR THE SAME SET UP FROM JET -HOT. mUCH SUPERIOR FINISH. ABOUT $1300.00 AND i PURCHASED HOOKER FREE FLOW MUFFLERS  ANOTHER $300.00. i DID THE INSTALL.a BIT OF A  PIA. NEED TO GET THE CAR UP HIGH, AND WITH AN AC CAR THE NEW BRACKET WAS A BITCH. YOU WILL MOST LIKELY HAVE AN EXHAUST LEAK SOMEWHERE. OF ALL THE GUYS I SPOKE WITH , VERY FEW WERE FURTUNATE ENOUGH NOT TO HAVE ANY LEAKS. HOWEVER, I WOULDN`T TRADE THEM IN FOR ANYTHING. AS FAR AS THE REAR VALANCE I BELIEVE YOU `77 EXHUAST EXITED UNDER THE CAR SO YOU SHOULDN`T HAVE THE EXHAUST HOLE ISSUE US BUMPER CARS HAVE. GOOD LUCK

I don't know if it is because I have the urethane setup on my suspension (as opposed to rubber) but after blowing 3 donuts, I finally went with lock washers and brass nuts.  They have held so far.

I have the Hooker headers/sidepipes from Jet-Hot and the Spiral Turbo Baffles also.  Here's my take:
1. They sound incredible, but very loud.  At WOT, you'd swear it was just headers.
2. You can lower the volume by plugging the center straight pipe that goes down the center of the baffles.  That leaves PLENTY of cross section so that you're not restricting air flow.  I did this with two tail pipe expanders.  Works well and I can take out easily any time I want.  Can't make up my mind which volume/tone I like best...
3.  Get heat shields or someone WILL get burned badly.  In my case, it was my son.  I also got the shields from cobrastuff.com
4.  The JetHot "Sterling" finish looks great on my '81, which has very little chrome and mostly polished aluminum.
5. VERY easy to install yourself.  If you can do headers,  problem.
6. The primary tubes are really too big for optimal performance on all but the hottest street small blocks.  The result is the low RPM torque will not be as good as it would be with a good set of 1-5/8" headers.  Higher RPM no problem.
7. Oh yeah, sidepipes can sound a little "tin-ny" compared to FlowMasters, for example.  I like the sound.  It's VERY aggressive (almost obnoxious) without the plugs I mentioned above.  However, if you want the sound of most aftermarket under-car mufflers, sidepipes aren't it.

Good luck!

[QUOTE=Stingray406SB]it's not so much "have to" as that I'm growing weary with having to spend money fixing things like weatherstrip, windshield wipers, AC, electrical wiring, gauges, all the little leaks, instead of spending money making it go faster.  not to mention it costs me twice as much because I don't know how to do any of it myself. [/QUOTE]

Not sure what you mean by "1974 side pipes".  If you mean factory, I have no idea about mods.  For the Hooker headers & sidepipe combo, I had to do the following mods:
1) cut a long 3/8" sliver off he bottom of the rocker moulding where the 4 header pipes come out under each side.
2) small cutout in rocker moulding to clear the bolt that clamps the pipe to the header
3) trim tiny amount off the bottom of the moulding to clear the two brackets that mount each pipe to the frame
4) trim even more to clear the clamps that hold on the heat shields (from cobrastuff.com)
5) trim just a little off the bottom of the fiberglass fender where the 4 pipes go under.
All in all, a few hours ot make the cuts nice and straight, rounded evenly, etc. to make it look good.  And, the rocker mouldings would now be worthless if I went back to undercar exhaust.
If I ever get this little project finished, I'll post some pictures. (I still have to paint the rockers and install them and the heat shields...)

Sorry I wasn't clear,1974 was the last year of the factory side pipes,I might have a chance to buy a set and wanted to know if anyone had installed the factory side pipes on a later year than 1974.

I put the factory ones on mine.  Small trim up by the door where they come out.  Drill a bigger hole in the frame and one behind it so you can get the bolt and your finger/screw driver in to hold the bolt and tighten up the nut to hold the back of the car.  Works better if you can lure a buddy over with whatever it takes.

If you have the factory rams horn headers, no prob., direct bolt-up.  If you have what we are all used to as headers, 4 tubes into a collector, you will have to have someone at a pipe bending and welding shop custom up an end for you.  As an alternative, you can use the "shorty" headers.  I used the factory rams horns that came on the car.   No problems and sounds great if you think LOUD is great!!!!! 

I would strongly advise lock washers and brass nuts as the factory nuts kept backing off and after blowing 3 donuts in 3 trips, I decided it was enough and even though everyone I talked with (didn't get to pulse the site folks though) said stay with iron nuts and no lock washers (even the books advise this) I figured what the he!!, it couldn't be any worse.  They have worked GREAT!!! no probs with backing off.  I got them at Ace Hardware.

Hope this helps!  You'll love 'em!