Im using a Bisson pitts smoke muffler, not sure Im going to do smoke though. Clearance under the box is next to perfect, with only a small amount to be cut out on exhaust side. I could not use the cupmount for the G-45 though. It pushed the engine way out front of the cowl. What Im doing is using the steel backplate that came with the engine. I then had to build up a an extension out of 1/8" pieces of light ply. I sandwiched together, then cut out for the backplate bolts and shaft. I now have the clearance for my spinner right on the money. Dont bother getting the cup mount! Its not gonna work! Oh yeah, had to remove the left side aluminum angle on the engine box to get the mount to fit as per instr. I plan to beef up the fire wall anyway, so I will mod slightly, or put the angle on the inside of the box. Not sure yet! I do not have the throttle set up yet, so Im not sure yet how Im gonna mount the throttle servo and run the pushrod.