One of the speed shop parts catalogs may still offer ‘locking’ manifold bolts. Once snugged down on the gaskets and locked with their central allen/hex bolt, no leak noise after 70K miles/20 yrs.for me.

6 VOLTS/POS. GRD. NW INDIANA

I'm no expert, but the 292 in my hot rod does the same thing.  Every few months or if I hear a "leaky exhaust sound," I'll just re-torque them.  

Someone here will likely have a real fix.  

I installed new exhaust manifold gaskets about 4 months ago (1955 Tbird 292).   I tightened them down and after running it a couple of times I tightened them again.   I checked them this weekend and a couple were not tight, they needed 1/8 of a turn.   I heard if you tighten them too much the manifold could crack.

I obviously have to go a little tighter, is this common?   I thought I read someone else had a similar issue.