Sideways is the best if you want to keep it simple and reliable.

I've used the Bisson pitts muffler on a couple of them when mounted sideways. I encountered the typical problem that most people have with pitts mufflers. Not enough pressure going into the tank. They lean out when you point the nose up. You can plug one side of the pitts and it'll run fine. I shaved a tank stopper down with a razor knife and stuffed it up inside the exhaust pipe. Put a little screw in the side of it to keep it from falling out.

They make great power on the stock muffler.

They make even more power with a pitts muffler and a Cline regulator. Muffler pressure not required. Free flowing exhaust and good fuel delivery.

I have one mounted inverted using the stock muffler. I had a custom exhaust extension machined to move the muffler out 1.5". I needed the exhaust extension to get the muffler outside the cowl with the inverted engine. Doesn't seam to effect power. It's on a Cline regulator.

If you want simple and reliable--just mount it sideways with stock muffler. But there are other ways to mount it and actually get more power or a better fit. Those other methods just aren't as simple.