Frame swap options

[rattle e 6speed]

I have been doing some research on the net about the pros and cons(or even possibility) of puttting the body from the 58 onto a 81-87 1/2 ton chassis. I know that the frame rail width is the same from the rear upto the firewall, but the newer frame converges inward where the front cross member bolts on. I know that the options for kits are endless, but $2-3000 seams a bit steep since you can pick up a running truck for under a grand, and end up with a roller. It just seems too easy, which usually means I am missing something obvious. Thanks for any input you have. Experience is invaluable, and can cost a lot to learn.