+1 on the Rick Ritter suggestion. I have bought 2 exhaust kits from him for out 250 trailblazers; he will set you straight on that! Google "ritter cycle". He is in OH or IL i believe.
He will probably tell you to put aftermarket piston in the 400; they are known for breaking piston skirts.
You may also consider converting to premix and using polaris gold oil; we've done that on all 3 of our Tb's and love it.
FYI the 'airbox mod' is simply removing the limiter in the airbox lid snorkel. You can do that before any mods