The big problem with cutting and splicing those two frames is that the 56 frame pretty well runs straight from the core support back to what little hump it might have over the rear axle. The square body frame this being a 77 1 ton has a big dip where it drops under the firewall.
This after I caught my breath after hauling that frame home on my 77 dualie with the 10 ft bed. How I drove from East Wenatchee to Toppenish without getting stopped is beond me.
With that one I am able to use the front crossmember under the 51 1-1/2 ton and with a bit of finessing use the rear springs and hangers on the slightly wider 51 frame. I've got a Dana 70 dualie rear axle for the 51 as it isn't intended to be a load hauler when done.