poly cab and core support mounts...I'm stumped!

[beetle1960]

Did a frame off on my 72 C10 LB. Installed new poly cab mounts followed directions included in kit and double checked correct placement here by reviewing past threads. The kit included core support mounts, i go to install fenders and there is a 1/2-3/4" gap bottom fender and door and 1/8-1/4" top of fender and door..like the front of cab is too low or core support to high..removed core support mounts and the core support is slammed to frame, but fenders now correct uniform gap between fender and door. Anyone seen this problem? Acting like I need to shim front of the can up by 1/2" plus.