The wheelbase will have to be the first thing to consider. Do they match? IS it close enough to hide any difference with fender trimming? Or do you have to modify the frame or axle location on the frame?
Physically joining the two together should be the easy part. Consider just torching the old body mounts from the frame and making new ones to match the Willys body, instead of modifying and reengineering the body. Might be easier, just depends. The headache is going to come from electrical, steering, brakes.... Chevy frames are shaped where it looks funny if there is too much space between the frame and body, so you might run into some headaches with the floorpan trying to get it to sit right.
That's gonna be awesome when it's done. A 51 Willys that you can buy parts for ANYWHERE.
