I don't know for sure on the frame lengths of the FW and the Deville. I am sure there is some members who do.
We have some juicers here, I don't know much on hydraulics, but I would think frame off is a good time to do it. Then you can beef up any areas on the frame that might need it for the additional abuse. These cars are pretty long, so the middle is the weakest point. Maybe if you are really enterprising, get a Suburban frame to work. They are 130 inches, very very close to yours. Front suspension is way different, and rear is leaf, but there is plenty of room there to put the 4 link back in, or just keep it leaf like the Suburbans do. (1992-1999 MY's) Probably a long shot, but the half ton frame is reasonablly beefy.
Becareful on lifting the body up, that is a LOT of weight to hold up. 5000+ lbs.