Weaver Scope Mount Question
 

I have an Ithaca X-15 semi-auto .22 with dovetail groves for Weaver-type mounts. The scope holds strongly right, so much that full crank-down left won't get it to zero (still off by 2-3 inches at 25 yards). I bought standard Weaver mounts (1"). Is it possible the dovetail is not quite square? Shimming for vertical would be easy, but how about left or right? Any suggestions? Since the receiver is narrow to accommodate the dovetail (it's probably 1/2"), I'm not sure there's enough base to tap-and-drill for regular mounts. Any ideas or suggestions would be welcome.

Thanks...