Wax you have a slight problem, the Mustang nose is raked and kinda steep at the bottom. Your best option is (if the ARF does not have the engine mount holes drilled and prepared on the firewall) to mount the engine at 135 degrees or 45 degrees downwards. The your pipe will be exiting at the bottom inline with the fuse. Try a Pitts style muffler, I doubt it will fit but then it will allow you to exit exhaust below but mount the engine on its side.