Fleetside bed installation

[64c20pickup]

Hello, I am new to this forum, I like it a lot, it is a good source of information for the restoration of my truck. I have a 1966 c20 that I am in the process of restoring I replaced the long stepside box to a new long fleetside box. there is something weird that I cannot explain to myself, when I put my new box directly on the frame, it falls lower than the Cab 3/4 "lower, yet the cab is seated on new pads only 1/2’’ in the front cab and directly to the support on back for a c20 ? Can someone explain to me what I'm doing wrong. normally what I can read on this forum is that we should not put a shim for the box, but I might not have chosen it

Thank you for help