Frame swap questions

[gikken20]

I have been looking into doing a frame swap on my 68 GMC longbed, I found a website for salvage vehicles http://www.salvage-cars-for-sale.info/ and what I was thinking was get a reg cab diesel longbed and put my body on it, the wheelbase for a newer truck is 133" I believe and from what I got off wikipedia is my truck is 127" for longbeds and 133" for longhorns, how do I tell if I have a longhorn, all the badging was removed from the PO and I havent seen my truck in 5 years, its at my parents house in OR and I will be getting it this summer when I move to Fort Lewis. Im deployed right now and all I have are a couple pictures. I kinda want a plan so when I get it I already know what im gonna do, Has anyone done a frame swap to newer frame? Would it be easier to keep it gas or would it be the same amount of work going to diesel? Ive done one frame swap, I had a 83 1/2 ton Chevy 2X4 that I put a 78 3/4 ton Chevy 4X4 frame under and it was fairly simple, everything bolted right up, but that was only 5 years difference not 40

Thanks for the help