I don't see a need to go to grade 8 bolts, as they're being threaded into cast iron and the extra torque needed to keep them from backing out might get close to stripping the threads out of the heads. I like to use star-shaped lockwashers under exhaust manifold bolts rather than the split-type locks.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!