I had the same problem with my GGR Diff mount as well, also the mounting plate wouldn't sit flat onto the remember there was a gap on the back edge of about 6mm is'h, I even tried loosening the beam and diff bolts to help make it sit flush but that still didn't work so I made a tapered Ali shim the same shape as the back plate to help it sit flush as I didn't want to bolt it up with the gap there incase it bent the base plate.

What I done to clear the ARB was to replace the top bolt with a countersunk bolt and also made a washer because the original back plate wasn't thick enough to countersink as I didn't want to weaken the fixing.

Here is some pics for reference, hope it helps

Tapered Ali shim






Bolt touching the ARB

You can see the gap between the mount plate and the cross member