My previous post here was about a muffler not fitting with the Evo 26GT2 in my H9 1.50 Mustang. If I modify the can of an inverted wrap bisson, in essence changing the shape of the can from an "O" to a "D" shape (viewed from the side) by cutting off rear of can and brazing a flat plate on, will I end up with greatly diminished hp? Probably only going to have to take off about 1/4" from rear of can. Cutting firewall is an option too, but a major portion will need removed and rebuilt because mod would put muffler right against gas tank. The muffler mod is an easy one for me, just concerned about performance. Waddaya think?

If the volume change is very little, there will be no large influence on power. 1,4" off a round section is indeed very little. Most of the cross sectional change is in the middle, not the outside. The proof of the pudding is in the eating isn't it?