The last Sweptline truck we did we just swapped the frame from a 78 Dodge 1/2 ton in place of the 70 1/2 ton frame. We didn't have any issues and it worked great.
Its a different time now, anything older then a 94 is gone from yards, and I'm not real impressed with the modern chassis. I guess the subframe would be a huge improvement, its just getting past the upfront cost. The strange thing is it doesn't bother me a bit to shell out $1,000 for a stand alone EFI harness. Must be the welder in me. Gene