You can use a pair of small automotive hose clamps as stops to set the length of this assembly. Start up the engine at low throttle, then advance to high throttle. You'll notice a very definite boost in power as the splat begins. If it doesn't either the propeller is too big or the pipe is too short. Switch to a lighter prop and try again.
Manufacturers generally make their pipes and headers about right or, if anything, a little long.
You should be able to get that nice boost without too much fiddling. The next step is to push the ends of the pipe and header together until they almost touch inside the neoprene coupler. This shortens the pipe a little. Don't let the header and pipe touch or the aluminum will grind away and be sucked into the exhaust port. Now, see if the engine still comes on the pipe while sitting on the ground.