I’m not going to stay out of which engine to use, but I will give you an alternative for a transmission and transmission adaptor.
Use an A833 overdrive 4 speed transmission and build your own adaptor.
The A833 Overdrive 4 speeds were installed in Chrysler product cars and Dodge trucks and vans from the late 70s thru the 80s. The early ones used a cast iron case, while the later ones had an aluminium case (avoid the aluminium case ones as the cases tend to crack. The A833 OD is based on the regular A833 transmission (that saw duty in such things as the 440 6 pac and Hemi cars) and will definitely stand up to some abuse. They are not as sought after as a lot of other transmissions so the buy in is usually not that bad.
Building an adaptor for one of these is pretty simple. You need the thin 3/4" adaptor plate that was used between the Hemi and Poly engines in the late 50’s. Just cut the lower legs off. You will need a flywheel, either an early Hemi flywheel (8 bolt) or a 6 bolt Poly flywheel can be used. Turn the flywheel down to the diameter of the later SB Mopar flywheel and install a SB mopar130 tooth ring gear. You’ll need a small block Mopar bellhousing preferably with the large bearing retainer hole and you’re all set. It takes a standard Chrysler started (with a 1/8” spacer).
On the Hemi you do need to check and make sure it’s already drilled for a pilot bearing (cranks used in front automatic transmissions generally weren’t).
Now the fun part, you can run some reasonably deep gears and still get good mileage and low highway RPM.
I’m running a Tri-Power 354 Hemi in my 57 Plymouth with a cast iron A833 OD and 4.10 gears (the OD drops the effective ratio to 3.00). It gets 20 MPG at 75 MPH with the AC on and I’m not a bit afraid to hammer it.
I discussed some adaptors in some old Hemi posts I did several years ago that might help.
http://www.clubhotrod.com/mopar-426-...-part-3-a.html
Just some food for thought.