FWIW, my 2 cents. This is just based on my experience from working in body shops since the 70's.
Based on what I've read here (glossing over it), here are some things I would consider.
Forgive me if I'm pointing out something you've covered.
First off, regarding wheelbase, when measuring at the spindle/hub you have to be certain both wheels are toed exactly straight ahead. Any deviation from dead on will make it appear as one wheel is ahead of the other.
Secondly, it's better to compare to rocker distance, not fender gap. Rockers don't move.
Third, what your last man pointed out about the cross member/cradle closing up is true. It was a common problem 40 years ago. We called it a "closed up" cross member. We had to put many GM cars on the frame machine and put pressure across the upper control arm mounting area of the frame (both sides) while jacking the center of the cross member up. This would solve CAMBER problems, not so much CASTER issues. Not saying yours doesn't suffer both, but I've included a picture of where I would be looking if I had a severe caster problem.
It's the rear mount of the lower control arm. If those are too close together it pulls the wheel back, shortening the wheel base. Below is a picture of a 67 mount.
A year ago we had a 70 Lemans in here with the same problem, but was drastic.
Anyway, good luck with it and if you need more measurements or pictures let me know. We have some GTO's in the shop with and without engines.
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