The surprise I have is that someone wants to swap frames from one with tougher gauge steel. Swapping a frame is a major job and I wonder what indicates you need to do this. If it is to gain IFS with a front clip I would understand the swap, but just because you suspect your frame has weakened???
A bunch of us here have done a "frame up" restoration on our trucks and I'll bet everyone would agree that the frame was as strong now as the day it was built. (exceptions of course make the rule). I would suggest if you're concerned about stress fatigue etc. you should pull the cab and check it out. If you are still concerned, box the frame and drive it another 60 years.
