Thanks Chris, I wasn't going to butt in and mess up a sale. Since you gave to go-ahead, here's my opinion:
Bryan, where are you located? That's an important consideration when shipping an engine.
You should be able to find a running 401 or 425 for under $800.
A 401 or 425 would be correct for a '63 Riv. The 425 would have some extra torque for a heavy car.
The easiest/most original swap would be a '63 425 (1st year of 425), or a 59(?)-63 401. Either would have the correct crankshaft flange, flex plate, and carburetor linkage for your Dynaflow trans.
A 65-66 with a ST400 trans would be a better performer but won't be original and will require more modifications to install. I don't have any experience with the Dynaflow, so I'll leave that to others.
What was mentioned was the Dynaflow crank flange, trans and driveshaft length, and crossmember. You will also have to change/modify the shifter, trans cooler lines, and then there is the possible issue of changing the engine balance if you change the flexplate that came with the replacement engine. (or did Dynaflow use a neutral-balance flexplate?....it was much different than the 64-up trans flexplate).
I'll guess your choice would come down to originality vs performance vs cost, and your ability to do necessary modifications or pay someone to do it.