THe 5.9 or 8 lt Cummins is an industrial engine so with the correct belhousing you can bolt any trans onto them , I have seen them in mid size trucks with 8 or 15 speed Eaton RR trans behind them , but you need an air compressor to shift the Eaton trans .
The 466 International is a great mid size engine also , very efficient & vitrualy any trans can be bolted up .
It looks like it has a split diff also , you might be able to change the hubs for alum wheels & retain the diff but swapping might not cost much either .
Air ride is the way to go .
They use the bigger 8 lt Cummins in Motorhome pusher chassis & they have lots of power , the 5.9 tweaked with an 8 speed should pull a 5th wheel with ease or with a flat deck should have lots of power .
Can't wait to see this get built