I have the Goldberg Extra, and the stock muffler, no matter if you shorten it or not, will stick out of the cowl too much. You are better off getting a BCM Pitts muffler for the 91 FX. It should have more power than the stock muffler.

Is this the mid-wing model? I don't think it's labeled a 300S, which is a low wing. I thought it's just called the 300.

On a different topic. I measured the width of the cowl. With luck, you may be able to fit the engine head completely in the cowl of you offset the engine w/ mount to the left of the firewall. You may need to do this anyway in order to get some right thrust AND to have the spinner come out at the centerline. Of course, this is assuming you side-mount the engine.

Good luck!