I haven't rebuilt a two speed but a number of 3 and 4 (OD) speed automatics. If you are willing to tackle a rebuild I suggest you get an ATSG (automatic transmission service group) manual for your particular transmission. These are good manuals and completely describe tear-down and build-up. Your local trans parts supply should be able to provide the manual as well as rebuild kits. If you can't find parts locally, places like Kanter might sell kits for the old powerglide.

The fun parts of rebuilding an automatic include removing some of those tricky inner and outer circlips (circlip pliers help a lot, replacing worn out bushings, re-installing pistons with inner and outer seals, and compressing clutch pack/piston assemblies to get them apart. If the thing is shifting properly, I'd leave the valve body alone. Only time you have to dig into that is if the trans is destroyed internally (metal pieces/fines floating around) or you want to change shift characteristics.