This may be the wrong forum...
since there are no religious or political discussions here, but why Royal Purple?
Any oil today you put in that transmission is scads better then lubricants of the 50's.
I've been an oil geek since the 80's with total belief in synthetics, I even send out used oil for analysis at least once a year. I've just never seen the RP people back up their claims...
Awesome looking restoration Greg, looking forward to seeing/hearing a video of it running!