To fix it properly you are looking at a huge amount of work and the rust will be back, guaranteed. Buy a "rust free" body from a dry State, and do a re&re of the diesel power. Then sell off the left over parts.
If you do it properly the rust should not come back. Two words - Cutting Torch
Cut out the cancer and weld in some new stuff
475,800 and rising
I am the black cloud of somke that flies by at 3500rpm!
(Please note diesel smoke is dangerous so lay off the throttle!)
Pulls like a draft horse, hauls like a pack mule.