The thrust line is way too high and the firewall has the right thrust built in, the mount has to be offset to the left. Also the firewall is drilled and has blind nuts installed for their mount. The only way you could get away with the muffler down the belly is to have a really long header extension and run it without a cowl. Best solution would be a nice Pitts style muffler if your field allows them. Here's a picture of mine. That is a lot of hardware hanging off the side.