Find a mount that is two separate "T" shaped beams with nothing connecting the rear of the two pieces. That will allow you to move the engine back about 1/2" further than the GP mount. Many ARFs use the two piece mounts and I think you can get an aluminum one from Sig or Hanger 9.

That won't leave room for the fuel lines to come out behind the engine, so switch to a Dubro, GP or Hayes tank and bring the fuel lines out alongthe outside of the mount instead of in the middle of the firewall.

Jim