Sheet metal covers develop dimples at the bolt holes, which compress the gasket in one small area, and leaves long sections without enough tension. Cast aluminum won't distort that way. Chevy finally got it right with their late-model engines, with four bolts down the middle like the old Studebaker, and a captive O-ring in a groove, instead of a gasket. Unfortunately, you'd have to change cylinder heads to take advantage of the new design! I'm not even sure the late heads will fit the old block, but I don't think so!
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!