You say that the intended trans is out of a 71 Ranchero? That would leave you 3 choices for automatics, the C6, the C4, and the FMX.
The C4 and the FMX have detachable bellhousings, the C6 does not. An FE C4 bellhousing is a hens tooth, the C6 would require purchasing a complete tranny for an FE engine, and if you have an FMX, I can help you out as I have an FE FMX bellhousing laying around .
If you have a Manual trans (toploader or 3 speed) you just need to track down the proper FE bellhousing. Good luck.