A buddy's '76 C-20 (350, 4 spd) would get stuck on hard dirt with a bit of sand on it. Very nose heavy with little weight over the rear wheels, added to an open diff, the poor breakover angle and lack of maneuverability makes for a bad off road vehicle.
My shortbed C-10 outperformed it all but carrying capacity.
I suggest you talk to
You can shorten the frame by cutting out a chunk from forward of the rear spring perches, around 18" to 24" inches, then section the bed to match. Relocate the fuel tank to the rear between the frame rails to aid f/r weight distribution and traction. Mount the spare in the rear as well.
I think the Class 8 guys quit trying to race a long bed forty years ago or so. And the desert wasn't nearly as whooped out back then...
Another old western rancher trick was to install a plate of 3/8 - 1/2 sheet steel on the bed floor to aid traction.
As for leaf springs, I'd suggest you speak with Deaver Spring Inc., too.
Website
902 E 2nd St, Santa Ana, CA 92701
(714) 542-3703
Might even save you some shipping costs and springs are heavy.
Old, but pertinent:
Deaver or National leaf springs?