Why do you need to weld the body, cut the rear panels and re-form the flange at the rear edge to re-mount the rear corners (which are steel), job done. The superstructurte is steel anyway so no problems.
The biggest problem with snubbing the nose is where to put all the c**p that's under there, the bonnet is steel although I'd suggest cutting the back edge of it away and the wings could be re-shaped to allow the headlights/indicators to be moved back. The only panel you need to cut and shut is the roof, you'll end up using a lot of filler on that anyway however you do it. Stary with the edges as that's the bit we all see.