new master cylinder or rebuild?

I just put MC36412 and MC36426 in on a p/n search on RockAuto. Both come up. The 36412 is '73-'76 A-body and '76-'77 F-body and '77 M-body, disc/drum. The 36426 is '71-'72 A-body, disc/drum. Both have 1-1/32" bore. Either will work for your swap. The 36426 probably has a residual pressure valve behind the tube seat in the forward outlet (for the rear drum brakes) so might be a better pick for a pre-'73 car, though at this late date it's highly unlikely the pre-'73 original rear wheel cylinders are still present.