Unfortunately, I cant answer your question, but I can offer another idea. I am doing a frame swap on my '92 ExtCab Stepside K1500. Took me awhile to find a good core and it was off a '98 that had minor rusting of the 2 rad mount pads. Also discovered the extra angular brace on the engine cradle is cracked, seemingly from a poor weld job by some UAW back in '97. I compared pics of a bare GMT800 and a GMT400. GM clearly re-used the layout but with improved/additional bracing in a number of key areas, BUT! Based on my personal experiences with GMT800's, GM cut the quality of the steel being used...There are tons of them that look horrible, far worse than any GMT400 did in the late 90's/2000's with similar age and miles. My guess would be that GM reduced the nickel content when the metal prices skyrocketed in the 2000's, (ie losing billions each year in the 2000's).
Anyway, my solution is boxing portions of the stock framerails, with opposing pieces from another GMT400 frame, adding tubular bracing, and welding up the stock weld spots the UAW workers missed/skipped. Just to be clear, Im cutting sections off the front half of a GMT400 frame and welding them to the inner sides of the base frame, passenger side section used on driver's innner, etc. The rear section is a bit trickier, because many are pretty nasty up here, so I might just need to have some sheet cut to size.
Bracing is fairly straightforward, Im pretty much copying what GM did to the middle section of the GMT800 frame, adding "flats" to the bottom of the spare tire "K" brace, and replacing all of the crappy formed steel U channel with tubular steel.
I also scored a set of stock '95-'98 GMT400 forged lower control arms from a K2500/K3500, and am contemplating the GMT800 front brake & knuckle swap.