Every Northstar I've worked on had round-headed headbolts (ten /head), and an Allen keyed hole in the bolthead that took a male hex (10mm)tool to remove. I think that's what the origional poster was looking for; the tool size to remove the bolt. The thread size is M11.
Now I'm thoroughly confused. Guess I should go out and check the FSM, but I was in the garage all day working on my wife's compressor clutch and am just to bruised, bloodied and battered to get up. I'll just stand corrected.