Honestly Bill, yours is the first problem like that I've heard.  Then again, I'm no expert and I don't have the Borgeson.

Hope someone gets the diagnosis right.

Not sure, since I haven't driven a car like this without power steering.  It feels more like a binding than hard turning, and it is not consistent.
If it is still bad after tomorrow's ride with the heat shield in place, I plan on getting another steering column.  I had replaced the original with a tilt.  When my mechanic put in the first Borgeson gearbox, the column was too long for the longer steering box so he had to cut 2 inches out and weld the pieces together.  I doubt this could be the issue, but at this point I don't know what else to do.  At any rate, I need to reposition the steering wheel and tilt angle so I have room between the steering wheel and the door for my knee to fit when I try to get my left foot on the clutch.  Six foot one and over 300 pounds is a bit of a tight squeeze in that area. . .

Thanks to everyone for their helpful hints.

It has been my understanding that to shorten the steering column, you don't cut it, you pound it in with a hammer, collapsing it the required amount.  If your mechanic cut and welded it, maybe there is an alignment issue.

In this case, it was the casing that was too long, not the saft inside of it.
I took the car out today with the fabricated heat shield in place shielding the steering box from the exhaust, and it still binds up.  I guess I'll get a new steering column (shorter than the one I used the first time so it won't need to be cut) and see if that helps.

What I can't figure is that if it is the steering column itself binding, why does it only start after 15 minutes driving on a hot day?

I believe there are some plastic pieces internal that are part of the collapsing of the column in the event of an accident. Perhaps these are expanding with the heat or something. That was why I suggested disconnecting the rag joint to see if the stiffness stayed with the column.

That is true for the stock column, but this is a generic tilt column that does not have the plastic pieces in it.  A 5/8 DD from the rag joint shaft rides inside the 3/4 shaft in the column.

FWIW, I've had my Borgeson box in for over a year and have not had any stiffness problems.  But I still do not have a lot of miles on it and have not driven it in extremely hot weather.  I actually have the opposite problem - boost is slightly more than I'd prefer.  @vettecountry recommended a washer kit to reduce the pressure, but I have not looked into it yet.  (Actually getting used to the feel)

Hi Dave, just bought another kit. I had 3 wachers and changed to 5. Did make a difference.

The new (Chinese clone) steering column came yesterday, so I pulled the one out of the car (another cheap Chinese clone), and on the bench you could feel a hard binding.  Not sure what happened, but then, I guess you get what you pay for.  These columns are less than $300.00, and a good ididit or similar will cost close to $1000.00 (ones without the key option are about half that price, but I needed the ignition key option).

In order to get more room for my left foot on the clutch, I moved the steering column closer to the driver.  The original column had the steering wheel in my lap, which was the main reason I went for a tilt column that was shorter, but it was too tight a fit between the wheel and the door for my foot to get on the clutch.

In order to move the wheel a bit closer to the driver with this new column, I had to move the ignition switch on the column down a few inches.  This ment drilling and taping new holes, and welding an extension to the rod that goes from the key switch to the actual ignition switch down on the column.  (I also had to fabricate brackets to mount it to the floor and under the dash.)

Today I got the new column in place, and now I have room for my clutch foot along with plenty of room between me and the steering wheel.  The only issue now is the flat connector for the turn signal and horn wires -- it looks right but does not make any connections.  The last column had a similar issue and I repined the wires to an old original GM connector for the column.

If I have to replace this column in the future, I will bite the bullet and buy an American made column -- which I should have done the first time or two ;(

Well, today I finally got the '75 on the road -- what a difference with a steering column that does not bind and with the steering wheel in just the right place.  I hadn't really noticed the stiffness in the wheel until after it got hot before, but now it is smooth and easy to turn.  The seating position seems so much more comfortable -- one wouldn't think just moving a steering wheel a few inches could make such a difference; it's like driving a different car.

I even managed to convince the eBay seller to refund the price of the defective turn signal switch (he could not have afforded to pay for the effort it took me to install it -- the path that the wiring takes through the column is bizarre).

Today is a great day.

If anyone is hesitant in switching to a Borgeson steering box, take it from me, it is night-and-day from the stock Corvette steering system.