The 6BT Cummins is a monster for a pick up truck. I'm not sure what I'd have to do with the coil springs in the front to hold that thing up. If it's the old 12V engine, the swap itself is pretty easy. It still has the mechanical pump on there so you only need to run a 12 volt wire to the shutdown solenoid. Rebuilding a 7.3L isn't much more than the 400 would be. There's only two valves per head, one cam, etc. The same configuration as the 400, just bigger parts. The kit itself will probably cost a few extra dollars, though.

Another option would be a Cummins 4BT engine. (four cylinder) Weight is closer to that 400 and it still has plenty of torque to get that monster rolling. It wouldn't be as hard on the running gear, either.