Body to Frame Shims (when topless door sags)

[Sandtrap9072]

I have a 1972 Gmc Jimmy and i recently installed a new poly body mount kit and now when i run topless i have to throw my shoulder into the door pretty hard to get the door open. i have been reading into this a little and it sounds like there should be a shim between the body and the frame,is this true? I now have the top on and the doors shut great. my question is, what is the best way to shim the body so that when i take the top off it is in the same position as when the top is on? My rocker boxes are in great shape and am thinking that not putting the factory shim back in is my problem. Any Help would be greatly appreciated.