Very ambitious project. I like it! Another member had a similar idea earlier this week. I modernized a 1957 3/4 a couple of years ago by putting a LWB 3/4 ton 1981 chevy 4x4 frame under it. I think it was a little easier than what you're attempting because Chevrolet widened the frames to 31.5" in 1955 and carried that through until 1987. Therefor, I was able to use the original body mount brackets off of the 57 frame and just fabricate vertical extensions to offset the dip in the Later frame. It also made fabricating core support mounts easier. The original frame of the 48 gets pretty narrow in the front. I said all of that to lead to some advice. I based all of my elevation measurements off of the bed. In other words, I positioned my bed where I wanted it, and pulled a series of straight line measurements to determine the difference in height of the frame to the various body mounts. That gets you close enough, and then you can use shims to fine tune it. the frame of the 48 is basically straight along the top of the frame rail, but the 82 frame will rise and fall, which is what allowed for the lower profile trucks of 1967-1987. I also had to shorten my frame by 8.5". I cut the frame with a sawzall, butt welded it back, and welded 3/8" diamond shaped gussetts on the inside and outside of the frame for added strength. I'll be glad to share any of my experiences with this type of conversion, just drop me a line. Sorry for the length.
