Yes you can cover the exhaust area with sheet metal. I would run the coroplast as one sheet across the rear suspension, leaving a big hole is not good. It's flexible and will move with the arms. The coroplast is attached to the arms with twisted wire, the wire is looped around the arm and through two holes in the chloroplast, holes are poked with a phillips screwdriver. The wire is twisted with pliers until tight. Big tie wraps would work too. My belly pan has been on for 8 years now that way. Pop rivets work if the exhaust sheet metal in on the inside, otherwise the ones for soft material, that flare when drawn up work. You can wrap the exhaust pipe with aluminium foil too. I had to add a few vent holes to keep the gas tank from getting too hot.
Yes, Check the axle ratio chart on the previous post for the mtx-75 transmission and your codes. There is also a 5th gear swap too:
http://teamzx2.com/threads/784-how-to-swap-5th-gear?