Only problem with these years is that changing to open drive means that you'll pretty much have to change everything from the bellhousing back. It's a big job, but if your stock parts all need going through anyway it's perfect time for the change. But, if you just want higher gears find a torque tube from a powerglide car, 3:55 ratio.
One major problem is that your 50 bellhousing to trans bolt pattern is weird, so for open the open you need a 55 or later bellhousing. Once that's done you have many more modern transmissions to choose from. A perfect donor car would be a 55-6-7, for the all the parts you will need.
Before you begin, give Buffalo a call (see his ad running on the home page). This man is probably the best in the country when it comes to knowing what fits what on these early chevs, plus he makes a zillion trick conversions for these cars to make this project really work in the end.
Best of luck
