If you cut 1 1/2 off the header to go inside the MCM then you did cut the header. The header length and the MCM big end tube are two different things. Get the brass tubing and use that on the MCM big end. A 1 1/2 cut off a header is a big big cut and would cause it to go lean. Whoever is telling you guys how to tune a pipe or MCM is uninformed. You are not tuning anything. You are mixing and matching mufflers and pipes and sometimes you luck out and sometimes you don't. A properly tuned MCM will make a 2 cycle engine run about 1,500 rpm faster and the midrange will be better than most 4 cycles and it will idle so low and so quiet you can't hardly hear it run. Also they will hold a needle valve setting forever or until there is a radical change in the weather. The time you are spending mixing and matching these headers and MCM could be spent flying instead of messing around with your engines.