First if the headers are hitting anything, that going to tweak them away from the heads and cause a leak. BTDT when I put a Lakewood bellhousing on my RR.

If clearance issues are ok, I've had great luck with soft copper gaskets at the head and headpipe (no leaks in nearly 20 years now). Jegs and Scummit have em. They're also supposed to be reuseable but I've never tried, like I said they havent leaked so no fooling and no retorqueing needed.

20lbs is about right for little 5/16ths bolts. Too tight will warp the flanges and or strip/break the bolts. Unfortunately you may have already done this. Not sure if your aftermarket heads have dry bolt holes or not but some thread sealer won't hurt.