Here is a more permanent fix better than shims.

1. Use washers to determine the best down/right thrust.
2. measure with an insidence meter etc..
3.remove the motor and mount from the firewall
4. keep the motor bolted to the mount.
5. use a belt sander or grinder to grind down the back of the motor mount to the proper angle.
6. temporarly install and recheck to ensure the thrust is as before, resand/grind as necessary.
7. reinstall the motor and mount.

Hope this helps