[screamingchief]

It's really not that complicated to avoid breaking thru if your goal is to not do so.

There have been more than a few threads here with folks posting their methods to gauge the distance/thickness of the bulge,one guy fabbed up a real nice fixture to do that,I saved a pic for reference it was so nice.

As stated most of these methods use some sort of solid round stock or such in the tube to measure the inside pushrod wall location when doing this.

But in the end if you go this route your still kinda at the mercy of the castings your working on as far as where the precise port widths are gonna end up as core shift may have one set of heads with lots of room and another set with next to none.

But there are a lot of good threads and info in the archives if you take the time to dig for it.