You could always do what I do...

Take off the mount and where it mounts to the firewall ( the flat side ) push that up against a belt sander, or any form of flat sander, then slightly tilt it as it sands some of the material away.

That way you can put in 1-2 degrees of corrective offset, while permitting the mount to lay flat on the firewall when you are done.

You can always throw it away should you change engines later, as they are relatively cheap compared to the effort in fixing all of this.

It's also far more secure and stable than shims and washers, since the force is distributed evenly against the firewall and mount.

This is obviously easier to do with plastic mounts...