Unless the action has been ground off, the scope mounts should take differences in actual height into the design. Many rifles have the rear bridge lower than the front ring and therefore the bases are different height(thickness). The rear base can be shimmed to level with the front base using aluminum pop(or beer if that's what you have) can material cut to match the outline of the base and drilled to match the screw holes. Brownells sells pre drilled and sized shim material of various thickness. On a serously over ground rear bridge, I've gone as far as to use JB Weld(epoxy) to form the spacer. What you need to be careful of is getting the rear base misaligned with the front base and bending, denting, or otherwise damaging or kinking the scope body.