Plugging smog holes on 302 heads

Jeebus Christmas!!! I just finished tapping & plugging the holes, thanks for the info, guys. For those who need to know, it is in fact 1/4-20. no drilling was needed for me, but don't blame me if you read this at one point in the future & break a tap off.

I made the mistake of mounting before plugging so it was a bear. Once I figured out the actual angle of the holes by inserting a hex key into one of them, I was able to match up the angle and the final 6 holes went super quick. That was a lesson I wont soon forget. Thanks again, all.