My '70 was an original 307 / 3.73 / 3 OTT truck. I swapped a 350 and a M21 and stayed with the bellhousing crossmember (no trans mount). Does your truck have that crossmember? Correct me if I'm wrong, but if you go automatic, wouldn't you also need to remove that to be able to install a "regular" transmission crossmember? Dirty Dingo makes a nice crossmember with height adjustability (for fine tuning driveline angles) and exhaust clearance, for lowered trucks. Here's a link:
https://www.ebay.com/itm/17297035555...YaAh_AEALw_wcB