Never real easy. If you're lucky, you'll be running the 4 screw-mounting version. If not so lucky, you have the three bolt mounting, and that will take three different size shims.

Basically, you must standoff the mount from the firewall using hard ply shims, or hard cf plate, or alum shim stock. Ain't no other way.

I've made a disc matching the o.d. of the mount, sanded the appropriately angle in it drilled clearance holes for the mounting screws and just thick ca'd that to the back of the existing mount.

It ain't fun, but usually you only have to take it in or out a couple of times. I'd suggest careful measurements, first.