missmatched brakes 64 gt

Did you actually drive the car yet with those brakes?

A smaller diameter wheel cylinder in the rear would definitely help and be totally undetectable. I believe what you are looking for is 7/8" (22.2mm) diameter. (Looking at a 1978 FSM, you most likely have 15/16" [23.8mm] now) I did this on my 68. May have been used in pickup trucks, not 100% sure. Not all wheel cylinders will fit the backing plates though. I modified a pair by milling them in a Bridgeport mill. Richard Ehrenberg's EBay store sometimes have them.

Residual pressure valve deletion will NOT help here. And most new replacement MCs don't have them anyway.

Sitting here on my couch , I just happened to look in a 1972 Plymouth Factory Service Manual. I see that a 1972 Valiant with 9" brakes uses 13/16" (20.6mm) diameter rear wheel cylinders. That surely would cut down the rear brake effectiveness. No inspector would pick up on that.

I don't know how you guys get parts over there, but Rockauto is a good source to at least look up specs.

I threw in the metric conversions for free.