What problems would I have using the rear springs out of
a 74 c10 under my 51 3600 it will have the front suspension from the same 74 c10 donor.
and planing on using the rear end from the donor as well

It's going to be a driver working pickup when done.
I don't want the ride or need the 3/4 ton capacity.
just removing a couple of leaves from the 3600 springs would be a second option.

But am I missing something or would it just be a matter
of properly locating the spring hangers to use the longer lighter c10 springs?