ZF 8HP90 like from the modern Challengers, Chargers, Ram trucks... stock will handle your HP/torque (come from factory behind Hellcats) and since it's a truck you likely won't have to deal with much floor pan surgery. The trannys themselves are not expensive and plentiful, just need a controller, adapter plate and yoke. You'd need an adapter plate regardless because big blocks never came from the factory with an overdrive transmission. Technically that's not entirely a Mopar tranny though, it was designed and built by ZF and installed in Chrysler RWD/4WD vehicles.
I like the idea of keeping the 4-speed and adding a GV unit though, IMO that's the most straightforward option.