Power brake booster size - Help

1965 valiant. trying to convert to power disk brakes. i have the rotors, spindles, etc. all on. i tried to just swap out master cylinder and that's not working correctly. Is an 8" setup going to fit in this car. also i'd imagine need to swap out the proportion valve also? needs some guidance here. was thinking of ordering the -
MP-203 1962-1972 MOPAR A, B, E Body 8" Dual Zinc Booster Conversion Kit w/ Adjustable Proportioning Valve
off of pirate jack. any input would be appreciated. thanks