I’m having a similar issue with my 77’ I believe it’s likely the toe settings but it could be a number of things. Only bad after 60 mph then it gets a little sketchy

Sounds alot like the front tires are toed in, instead of towed out. or its changing toe as you are driving. I had this happen at full song on the back straight of Road Atlanta and it scared theliving crap out of me: any slight input to the wheel and it veered either way... virtually no control.