I recently mounted a Moki 1.8 in quarter scale and found that a two piece beam type fiber mount allowed me to move the engine back to where the engine case was almost touching the firewall. The mount I used came from Sig. Maybe you can cut the center section of the Greatplanes mount to allow the moter to slide back even further. Once bolted to firewall the material between the beams of a mount is pretty much useless anyway. Also maybe change needle valve mounting from engine to airframe.

I have always had great results with bisson muffs...so has everyone else hear...never one problem and I am running quite a few to say the least.