Just want to add:
If the R is a later model, it will have the large bolt pattern. I don"t know what year they changed. I"m pretty sure you can put the ZT hubs on the R axle and it will work, though. Or you can swap axles between bolsters.
Also, I do know that a pedestal from a ZA had a longer steering shaft coming out of it. This probably is not an issue in your case unless the donor R has the Eaton or Webster hydraulic pump in the side cover.
That round hole with the plug in it between the wheels is probably a core hole, the core to mold the inside was supported through it. It is not on the later steering columns.