I believe the PB assy has it's unique brake pedal rod that's different from the manual brake one. The 68 disc/drum M/C iron will work. A later alum M/C will save a slight bit of weight & requires a 4 bolt or 4 hole to 2 bolt alum adapter and Dr Diff can sell (or recommend) one with the correct bore size. No difference in the performance between alum and cast iron M/C's. You MAY require a different proportioning valve. Easiest/cheapest would be to get the above 68 MC & your missing PB rod & install it & bleed it out & see how it acts.