Have purchased 15"x8" Crager SS for the rear of my 1964 SS. Any suggestions on tires that will fit without alterations. Would like to go as wide as possible.

Hard to answer that question without knowing the backspacing on those wheels.

Sorry.. The back spacing on these are 4 1/4"

Since the optimum backspace for a 15x8 wheel on a 64-65 is 5", that 4.25" backspace is going to limit how large you can go.