The holes in the main bearing cap are 5/16 already. you'll need to go to 3/8" to get any threads in those holes. If the thrust plate screws are out, the cam should slip out with a little persuasion. I've installed several timing gears without removing the cam- - - -that's just the way it was done back when these things were daily drivers.
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!