what trans should i buy for my 72 old with a stock 350 motor

Your car probably had a turbo 350 but you can probably add a 4l60/TH700-R4 for 4 gears instead of 3.

This article might help you ------------------------- http://www.superchevy.com/how-to/46140- chevrolet-transmission-swap-guide/

All depends what type of driving you want to do. A TH350 or TH400 were standard then for the higher horsepower motors. Yours likely came with a TH350. A check of the VIN or data/build sheet will show that.

I very much doubt you would find a 4 L 60 E or even a 700 R 4 that would fit because if it's a true Old's motor . Transmissions and bell housings are different from Chevrolet or late model GM ( Pontiac Old's Buick ) a transmissions ( 350 / 400 hydromatics ) will fit as you can tell by my handle I've done a lot work on Olds

Summit Racing has the adapter you would need. Part #0061