I think it'd be so much work trying to remake the frame with tubing unless your one of those people that just wants to run it straight instead of factory bends... lol then it won't be too hard but i like what GM does with their stuff but this here isn't looking too good you might want to check the rest of your frame but... if it was me and i didn't want to get another frame etc etc...
As a welder you should also be a fabricator
so here's what a fellow fabricator/welder would do the long long way..
I would cut the frame right behind the cab and get 3 - 3/16-1/4in thick plates depending if you want to match a 1/2t and 3/4t frame thickness or upgrade to the 1 ton frames then go with 1/4in. The 3 plates would match the 3 parts of the frame top frame rail, middle and bottom. Clean the old frame up and clamp the new top piece to it and heat it up and bend it to contour the frame then the middle plate i would put over the old part of the middle frame and cut it out to match the bend of the frame, then repeat the bottom rail as you did the top rail. Put them together corner to corner obviously then square it, tack it. Slowly weld it all together. Maybe throw some gussets in there lol
But that's just how i would do it just because i like matching original stuff im not big on tube framing etc etc but just me nothing against all that or just do like everyone else says and buy a new frame or something
1/2-3/4t = 3/16in
1 ton = 1/4in and taller
I can get pics of both if needed