'65 GT Manual Steering Overheating?

The steering on my '65 fastback seems to tighten up after I have been driving for a while. I noticed the problem after I bought the car. I had the original gearbox rebuilt about 2 years ago and it took all the play out, but it still tightens up after some drive time. I also rebuilt the front-end. I have two theories:

First, the gearbox sits right next to the exhaust manifold and gets screaming hot after I have been on the road. Anyone ever here of this creating a problem or the use of a heat shield?

Second, as the tires heat up they grip the road better and are more difficult to steer. The car has eagle GT IIs. I don't recall the size. They are a little wider that the original tires, but they are not overly meaty.

Any words of wisdom? Thanks in advance.
 
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