I just bought a garaged 82 F250 XLT Lariet,C6 with 76 K documented miles from the original owner. It has 400 ci engine that wont turn more than 1 inch at the crank. The truck has the trailer package and some other options. Also a stock Dana 70 rear. It had a 5th wheel. It has absolutly NO rust. The bottom of the doors are spotless. I was going to use it as a parts truck but its too clean.
What I want to do now is to replace the engine an find a f350 4x4 drive train and do a swap.

 

It might be worth it to just free up the motor.

 

I hear the 400M engines are garbage i think the bellhousing bolt patterns on the 351m 400m and the 460 are all the same id put a 460 in place of the 400m i think your best bet as far as a 2wd to 4wd swap is find a 250 or 350 4x4 complete truck thats cheap or just the frame and running gear and swap your cab bed and everything else onto it just my opions though but i think you should ditch the 400m for a 460