I'm Cummins swapping a '74 Duster, thought I'd share the progress.

I like it, it's going to be a challenge but it sounds like you're prepared to go the distances needed to make it work. If I were to do something similar I'd take the approach like some others mentioned of shortening a truck frame and grafting the Duster body onto that, OR beefing up the Duster unibody to have similar strength to a 3/4-ton truck. But then you don't intend to tow so maybe closer to 1/2-ton is sufficient? Idk if it would make much difference since a lot of it will need to be custom fabbed anyway.

I do know from my own research into finding ways of getting a Cummins into my '72 D200 I've learned a lot about the 1st-gen Cummins trucks (1989-93) which were the same platform as mine. Another transmission worth considering might be the Getrag G360, they have a mixed reputation due to some flaws that made them seen as weak/unreliable when new but over time people have figured out how to make them last and there are some aftermarket parts out there to beef them up a tad. Supposedly they are just as strong as the NV4500 and externally rather small compared to the G56.

You might gain some knowledge spending a little time over at Ramchargercentral.com. More than a few guys have swapped 6BT's into RC's and TrailDusters which are on 1/2-ton frames; apparently boxing the C-channel frame rails adds a lot of strength which you already have with the Duster unibody framerails, they just aren't as tall or as thick gauge of steel. Welding plates to the frame rails, radiator support, firewall, and adding beefy subframe connectors (duh!) might get it stiff enough to not turn into a pretzel when you get on the boost and unleash that quad-digit torque. Also being an inline engine you'd (in theory) have more room to add down bars from the firewall to the front frame rails to help support all that weight with a coil-spring front suspension. I'm not sure I'd feel comfortable with a typical coilover conversion front end though unless that also got extra beef added as well.