If your rear brakes are indeed the '87-'88 T-bird / Mustang items, then this explains why your current front brakes are working so much better than the rear brakes are (as the little itty bitty Fox body rear brakes just don't provide that much effective brake torque). We have done quite a bit of back to back testing with brake line sizing, finding no particular difference between a 1/4" and 3/16" brake line (main line running the length of the vehicle to the rear brakes). I suggest you swap out the rear pads to a more aggressive compound as the next logical move:
https://www.summitracing.com/parts/e...172r/overview/