The muffler should not cause any problems, we mount engines in all kind of configurations and the mufflers hanging out in the slipstream don't do anything. (Keep in mind, it's at the front of the plane so would have less impact anyway.)

The mount should definitely be attached to the firewall with machine screws and blind nuts (also called T nuts I believe). Many people say using "wood" or "sheet metal" screws to attach the engine to the mount work fine, but for me, I either use bolts with nuts, or tap the mount for bolts.

If you can lower the tank you'd be better off, but I don't think it will be a huge issue. You generally want the centerline of the tank even with the carb needle, or as close to that as possible to optimize fuel draw, but prevent siphoning. (If the tank is too high, once fuel starts flowing it will keep flowing, even without the engine running.)

Have fun bashing the 4*. They're a great flying plane, but a bit lacking in the looks department. (Just my opinion! )
Dennis-