I remember seeing a factory number for this, was 0.62/0.68 inch or there abouts. Most difficult chore is getting the thing to move at all. I've heard all kinds of methods from heating the area to drilling the plug out and threading the hole for a plug. If the motor is out of the vehicle I'd take it to a good machine shop.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed