I would think (and numerous truck folks seem to agree) that your second method of sliding everything forward then chopping off the rear would be more sound, only because you would still have your framerails uncut or modified (except for a few extra holes, which are NOT a concern here). No matter how good the welding would be, to me it would be better not to have to weld at all. Just one less thing to worry about! I hope this helps you. Best of luck to you!