Brake push rod

Lucky you salvaged the rubber bushing which locks the brake rod in the MC piston. I did a test fit with a new bushing once, then had to remove the rod (to slip on the dust boot) and tore up the new bushing getting it out. I bought them from a brake vendor (Dr Diff?). My 1964 Valiant is manual drum brakes, but I changed to a dual MC, using an aluminum 2-bolt one for a 1999 Breeze ABS ($25 new) with a 2-4 adapter plate ($30 ebay). The brake rod fit and the pedal wound up in the same place. I glued a dust boot to the MC to be safe.
Manual Brake Booster Rod? Conversion for 65 Barracuda

Many use the Dr Diff package, which leverages a later Dodge truck MC. I recall a version with a smaller custom bore. But some prefer the old crusty cast-iron with leaky cap. Mopar Aluminum Master Cylinder Kit

For a conversion to front disks, most people use a proportioning valve. If not, the rear wheels may skid first, to cause loss of control. I put a new adjustable prop valve in the rear tubing in all my old Mopars. Adjust in a wet parking lot until the fronts skid just before the rears. I kept the OE distribution block, but just as a tee for the 2 front wheels. Plugged the rear port (inv flare M plug) and put a 3/16"F to 1/4"M inv flare adapter in the top port from the MC (F port, at rear), all fittings in bubble packs at Autozone.