Mike, No overdrive four speed automatic bolts directly to the Nailhead bell housing. It will require a bell housing adapter and extension for the flexplate bolted to the crank:
like this. Then your choices are the 700-r4, 200-4r, 4l60e and 4l80e. Some of the early 4ls have stand alone computers, but the 700-r4 and 200-4r won't require a computer. Any one of them will be need to be built for or adapted to your use. Any of them will handle your engine when this is done. Once you have selected, you will need to remount the crossmember, get the correct transmission mount, shorten your driveshaft, acquire and install the correct yoke for the transmission on the driveshaft, work out any exhaust clearance issues,
install the correct shifter kit and cable, use larger 3/8" transmission lines (may have to bend yourself) and figure out any TV cable or computer wiring or mounts.
Forced to choose, I would use a built 200-4r for a 4 speed, but personally I would use a three speed ST400 from a 65-66 Nailhead used in the big cars. You already have the wiring for a switch pitch trans, your converter would give you a little more stall in the ST400 and it bolts up to the Nailhead. You still have to do some of the fitting, will need driveshaft shortening, the correct yoke and lines and shifter modification.
