Swap from a 4-speed to an Auto

I know someone running the stock O/D gear set in an A833 behind a 440 6 pack. Not a problem. I would not power shift that into O/D, the O/D is for cruising on the highway. I have been running an early O/D for years behind my HP 273 and it runs like new. I also run 90 wt gear oil. I like to use the gear set and main shaft with a standard cast iron main case, 64 or 65 if I can find one. It is stronger with no "float" and uses the standard front bearing retainer so I don't need any special bellhousing or adapter rings. Some of the later O/D's have cheaper cast gears and funky side covers and internal linkage. If you are making a lot of power, 3 speeds are enough + an O/D cruising gear. You can get an O/D main shaft and gear set relatively cheap so they are economical to put together.