I can tell you from first-hand information that 205/70-14 on 6″ wheels fit. I can also tell you first-hand that 215/60-15s on stock 15″ wheels fit. Both of those combos are about 25.25″ in diameter. Your 205/75-15s are about 27.1″ in diameter. Those would be a good choice for a 1968-72 Cutlass, but are waaaay too tall for the 61-63 cars.