Marc,
It is really not that difficult, this is my 3rd shorten job. The entire frame is leveled on a combination of jacks and blocks. Just make sure it cant move. Then everything else is simple... just measure 4000 times and cut once. measure 4000 times again and weld once. measure another 4000 times just to make sure it is exactly where it needs to be. Reinforce the hell out of it, boxed it etc. Simple huh
Jason