The MACS instructions are for their tuned pipe installations, not for header pipes alone. Leave your MACS header the length it came at, which seems rather long, but your not putting a MACS tuned pipe on the end of it. The mousse can acts as a low restriction muffler. It has very little if any tuning properties.

The mousse can muffler works very much like the tuned mufflers that come with Rossi, MVVS, Nelson and Jett motors. It's very similar in function to a header pipe on a 4-stroke. It helps scavenge your engine by "extraction". This is very different than a complete MACS tuned pipe (also known as an expansion chamber). A complete tuned pipe will send a positive reversionary wave back to the exhaust that helps prevent fresh fuel and air from being "over-scavenged" into the exhaust system. At the right RPM an efficient tuned pipe can actually give a slight supercharging effect. Because of it's properties the tuned pipes work well with engines that have long exhaust port timing. For regular sport engines with less than 165 degrees of exhaust duration the mousse can style muffler is actually very effective, especially when the header is left long and larger props are used, like 11x6's on a .46.