I had intended to use a known unsurfaced LA head as a pattern, and index the jig on the top of the head, as tops of the head bolt holes. I've forgotten, I had taken some prelim measurements between some junk heads, and the measurement of the tops of the head holes to the surface of the head were fairly uniform, as I recall. I don't remember, I guess the Magnum head bolt "height" is different so you'd have to adjust the jig to that.
