Good question... I did not consider that but I think you are right! Thinking about using the TTI down pipes in the 3" size and then 2 1/2 after the x-pipe to the tailpipes... If I have to grind out the head pipes to 2 1/2 than I guess I will have too..... I will get with TTI tomorrow and ask them the inside diameter of there headpipes.... I already opened up the exhaust flanges to the 2 1/2" size so I may have to fire up the ole makita die grinder and let it rip on the headpipes when I get them! TBF