I really like the GP adjustable mounts. In fact, I prefer them to most other mounts that I have used.

Here's how I install them:
-Put the two halves together (not on the plane) then adjust it to the correct width for the engine and tack the halves together with medium CA.
-Bolt the mount to the firewall
-Place the engine on the mount and determine the correct distance from the firewall and hold it there with a spring clamp or two.
-Mark the location for the engine mounting holes. The great planes dead center tool works great for this.
-Remove the engine then remove the mount from the plane.
-Drill the engine mounting holes in the mount using the proper size bit and tap the threads.
-Install the mount on the plane for the final time using blue loctite on the screws along with a flat and a lock washer.
-Bolt the engine onto the mount using the holes that were just drilled & tapped. Be sure to use a flat and a lock washer here.

If you take your time and work accurately it will come out perfect and be hassle free.