The factory option on 3100 was an additional 2 springs to the leaves-pack.

http://chevy.oldcarmanualproject.com/chevyresto/54134.htm

Basically the same 1954 3100 frame on all trucks, except for the panel-body "hangers"/plates.

There are wood mounting blocks under the rear of the 3100 rear panel-body trucks' bodies. I do not know if those blocks were used/needed in earlier years.