The headers need four slots. The muffler has four.
3-28-08 It's just been reported to me the you can use a belt sander (hub down) and reduce the bell of the header to make header length adjustments. That helps.
I cut new slots less than 7/64ths deep and finished them with a needle burr to a 7/64ths depth. The exhaust spigots are about 5/8" long.
It's easier to start a grit-blade if you make a couple of rim divots. WD-40 won't wash away Sharpie® ink. Make the same marks inside the pipe as you do on the outside. A new Grit-Blade leaves a 0.060" kerf.
With a new Grit-Blade you can cut both slots at the same time.
Stop cutting early.
A pin burr rounds the bottom of the slot. Might help keep it from cracking. 7/64ths is plenty deep, using four slots.