Best Swap Transmission?

My first choice would be the modern 8 speed automatic. If you're going this far, and keeping the car for awhile, may as well go all in. Good acceleration in the lower gears, almost imperceptible shifts, and good cruising gears. Sound German has you covered, ask for his recommendation.

It's big, but your C-body has a pretty big tunnel. I've seen used ones going for less than $1000 from wrecked Challengers/Chargers. You don't need the Hellcat version.