Run two nuts onto a single stud until the outermost one is 1/4" past the stud's end. Tighten the two nuts together, then put a wrencn on the inner nut and screw the stud out. Put some penetrating oil on each stud the night before to help smooth the removal. Installation is the reverse process. You might want to put some loctite on the studs before installing and perhaps even chase the threaded holes in the head with a tap for good measure. 20 ft lbs sounds like plenty for the stud bolts. Be sure to use the special thick washers where called for.

I bead blasted my manifold, then submerged it in mineral spirits to ensure all grit was off before painting. If you sand between coats, you should get a presentable finish.

Regards
123 in PHX