I've managed to make an exhaust header for the .061, both side exhaust and rear exhaust allowing a bolt on set up. Both configurations respond well to a tuned pipe. I gain 3K on a 6 X 3 and 25% fuel using a CS (China Sang) pipe.

I've no doubt that the .074 will respond to a pipe as well, and the plus is that the bolt on feature is ready to go. I've adapted the muffler to use a long piece of Flyquiet tubing on my .074s and with a baffle in just the right place I gain at least 500 rpm. The engine is quieter and the plane is cleaner as a bonus.

I suppose all you need to do is size the plane according to the size of motor. All things considered though, the .074 looks like your best bet.