Ouch that did take an impact.

You will find that the fuse box wont fit properly as the 909 shells used the later saff bulkhead, best thing to do is to remove the outer case of the existing fuse box and swap it for any mk2 sierra version, that way it bolts to the shell properly and the fuse board still sits nice and dry inside it. Also you will need a Mk2 sierra internal rear courtesy light in the boot and drill the hole for the rear aerial. Your rear tow eye will also sit a bit lower than the existing one, no biggie though.

Resell value, well I can tell you for a fact that over 25 RS500s use these later shells now, and they seem to sell fine regardless.

I would rather a nice fresh rust free shell over a bodged up original one all day long.