If that is a practice head and it’s really scrap, don’t be afraid to go too far to “see” what that looks and feels like. Practice on the short turns. Keep shaping the guide boss down until it “looks” wrong. Blow a hole through the main bolt boss that is in the common wall of the intake ports. Now that I said that, I’ll need to go out and see if I blew through my iron heads. That may not happen on iron, but it will on aluminum heads most of the time.
Going too far now will help you learn what it is when you are doing it for real. Don’t worry about grinding through the pushrod pinch too.
Now is the time to experiment.
Also, if you don’t have a sonic tester, a 1/8th inch drill bit is your friend. If you aren’t sure how thick something it, drill a hole in it. You may be surprised how thin some of this stuff is.
