Modern Drivetrain Swap into '65 Valiant

In the spring I acquired a '65 Valiant wagon. The exterior shell seems to be in fine shape, but the floor is completely shot. The motor is locked up and I wouldn't trust the transmission either at this point. Since I'll have to complete major metal repairs as well as a complete driveline swap in any case, I would like to go with a good modern overdrive transmission.

The transmission is the focus here, as driveability is my priority with this car. I would like to keep a small motor in the car - no v8, although a 273 would be cool. It would be nice to keep the slant 6, but it doesn't seem like there are any straightforward overdrive options other than a 4-spd. Budget and simplicity are key. I was thinking about putting in a jeep 2.5 i4 or 4.0 i6 with an ax-5 out of a mid 90's Cherokee. Plenty of power for a small motor, and lots of support for the jeep guys. It would be nice to have a T-5, too. I could grab the engine and trans out of an S-10. Of course, maybe the work required to move away from the 904 isn't worth getting rid of the slant 6.

What do y'all think? What good motor/trans combo would be a good replacement for the factory setup?