What is the largest rim that will fit on a 1975 Cutlass Supreme without altering the fender well or damaging the car in any way? (And sorry to those who responded to my previous thread, i put in the wrong year of the car).

Be more specific. Do you mean width or height?

Realistically, you would be looking at a wheel and tire combination.
IMHO 17 x 7 up front with a p235/50
and 17x8 with a p245 would be the reasonable limit.
Anything bigger will look too rediculous.