That car has a torque tube diff, so if you want to swap the trans, you need a new rear end. 2nd gen Camaro rear will fit up and it will hold up.. If you just wanted a better driver and a stock 3spd, then you could just get the rear out of a '51-'54 Powerglide car as PG cars had 3.73's and stick cars had 4.11's
A 700R4 Auto is a good auto, but the 235 if in stock form is going to 'feel' overdrive a bit. I would keep 4.11 gearing if you went with the OD trans.
My car ( '51 Fleetline, 235/ Powerglide ) will go down the road about 60 MPH with more to spare ( 60 is as far as I'v taken it ), but it cruises best at 50-55 MPH