pull prts from spindle on out from any dorno GM A-body ( think 64 to 72 ) may want to double check years also master cylinder and booster if applicable, I'd steer away from the used proportioning valve and get an adjustable aftermarrket unit , as you may have some issues w/ brake lock up due to volume of fluid , etc. at rear wheel cylinders. This probably will place the centerline of your rimss ( may not be able to use all with discs ) at a different position ,so that is another thing to consider. or go full aftermarket route from any of several vendors. donor car route is (can be ) cheap Full set-up cost me $85 but lotsa work and expenses can still add up may need new bearings ,seals, etc....and fairly labor intensive aftermarket route ain't cheap at all and can still be lotsa work all depends on your skill level and inclinations...
