1964 Plymouth Valiant Leaf SPring Relocation Kit, How big of a tire can I go ?

That tire showed as a 275/60 R15 with a full relocation kit and mini tubs you should be good to go. If you stick with a stock width rear set the tires up then measure for wheels.

Unless you are set on a particular wheel with limited sizes then set up wheels and narrow axle to fit.

My 66 Dart with mini tubs and springs moved can fit a 325/50 R15