Why limit yourself to only the ‘64 & ‘65 models with the ball and trunion output shaft. You could use the ‘66 - ‘75 splined output shaft transmissions by replacing the driveshaft or having a new driveshaft made.
I am only for ease of install. If I can gather all original parts for the swap, hopefully, will go quickly and smoothly. BUT I guess if a later model Trans is offered to me at the right price I would definitely consider.