guy, the quick answer to your question is 115" vs 127" wheelbase.

All the "extra" length is in the "middle" of the frame. Shortening a frame is not that big a deal if you remember the basics, have excellent welding skills, reinforce the cut section, and are accurate in your measurements and cuts. There'll be a bunch of "little" things that need to be dealt with e.g. X member modification, shortening of brake lines, e brake cables, drive shaft, etc. Don't start the job without having the shorter bed as you'll likely need to move the bed mounting brackets.

Personally, I'd put the extra effort into finding a SWB frame and bed and transfer my LWB stuff over, and then only if my LWB stuff was so much better than what would be available otherwise. And you didn't even mention whether the LWB was a 1/2 ton or not. If it's a 3/4 ton I wouldn't bother. This is one of those kind of projects where you're almost better off selling the LWB rig and finding a SWB to replace it with. Especially if there's going to be a lot of body work to be done on the existing fenders, cab, etc.