Tires

Exactly. If you have the factory wheels you really can’t go much wider than the 205’s you already have because of the narrow factory rims.

I’ll add that if you have aftermarket wheels you can run up to a 245 in the back if the backspacing is correct. That will depend on the axle that you have and its bolt pattern/brakes. A ‘75 should have a 7.25 or 8.25, which are the same width, but your ‘75 could still be SBP if it’s drums all around or BBP if it’s a disk/drum car, and that does change the backspace needed. And of course, someone could have swapped something out by now so it’s always good to check. 7.25’s get swapped out all the time.