i think i drilled the holes first but i don't think it really matters. either way, make sure you hit the threads of your screws with sealant before you sink them.

AND, don't tighten everything down on day 1. tighten it down about 75% of the way, then on day 2 snug it up some more. if you tighten it down at first, you run the chance of squeezing all of the sealant out. by waiting until day 2, the sealant will create a better 'gasket' when you tighten the screws.

don't forget to seal where the trapp and exhaust tube meet...about 2" inside the pipe.