Basically, you'll need to make sure the engine is fully broken in, then set it up on a stand or your plane or whatever.

Run it with no muffler, no header and get a tach reading at full throttle. (Put your earplugs in)
Then put your header on, and your pipe. Make sure there's only a very little gap in between the pipe and the header. Fire it up and run WOT. The RPM should be close to, or a little less than the reading you got with no muffler. Cut the header little by little...and each time fire it up and run WOT and note the tachometer. The RPMs should go up with each cut...until you've gone too far. Then you'll have less power, and an overheating problem...so be careful.

If you get to a point where your satisfied with the extra power, stay there. Otherwise, you may end up having to buy a new header and start all over.

You ought to use the same fuel you'll be using all the time too when you're doing the tests