Assuming you have a 161" wheelbase frame? Then it's the same as the 2 ton frame and I think is plenty stout in stock form. at least to me they look a fair amount more substantial than the dodge truck frame that came with the Cummins...
Not sure I like the idea of making 6"x? rectangular tube into air tanks. Most all air tanks you'll find are round for a reason.. that's the shape they want to be.
I expect you'll have plenty of room for air tanks under the bed and above the frame.
Also look at some of the air tanks on semi trailers, they can mount between frame rails left to right, and are pretty cheap to get new.
I would not box the frame, both for your reasons and the risk of cracking because it's to stiff and or poor transitions leaving places for cracks to start. It's also difficult to run brake lines and wires and such.
Grigg
