Like John, I used a hi-heat Permatex as well. And ya, it's a good thing you picked up some new O-rings. I lucked out and mine were all still in good condition and didnt tear, so I re-used them. Had a bag of new spares on-hand in case.
May as well get some new coolant too. A fresh flush never hurts. Hell, you gotta drain down anyways.
Oh, and I am sure you know, but clean the plenum very well after drilling. Metal shavings BAD.
Good luck and have fun.
EDIT: Damn John, seeing your post just reminded me.... I forgot all about those pillar covers! I'll holler at my mom tonight (since they are in her garage, heh) and see if she can ship them out to ya. I still got your info. Sorry.
Gary Noonan
'01 S/C VX / '18 Forester XT