Stainless into cast iron shouldn't present a problem, particularly if it's lubed with something like Never-Seez. I like the star-shaped lockwashers, as I think they have more biting edges that a regular split lock. That flanged bolt with a star lockwasher under the head would probably hold about 3 days past forever! Yes, use the 3/4" long bolt - - - -the more thread engagement the better!
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!