does it have a switchpitch? i don't think they were supposed too in 1963.
you need to be aware of RivOwners.org, lot of help there.
The trans needed a lot of odd-ball hard-to-get stuff. I'm having it rebuilt but wonder if I should've tried to swap a modern overdrive in there.
stock trans should be a Dyna-slush. even Buick started using Super Turbine 400s in 1964. that's basically a TH-400 plus a switch pitch convertor. would be MUCH sportier than the Dyna.
Nailhead bell pattern doesn't really work with anything else, so if you're going to use an adapter plate you should just use whatever trans you want.
iirc, there's something nutty like AMC using GM transmissions with a Naihead pattern bell housing AND an adapter to all of their engines that they were using it with. so you might be able to find a TH-400 with a Nailhead bell in an AMC or Jeep from the mid-60s to early 70s.