12 Second N/A Slant 6?

Valve retainer seats can be deepened on the head casting.

I want to keep as close as possible to the OEM rocker position @ .000" lift of praotical.

You should go on slantsix.org and look. Doug Dutra....or maybe it was another member.....anyway somebody over there cut a slant head up and they are very thin in that area. It was advised NOT to cut the valve spring seat. But if you decide to, I wish you luck. ...and again keep us posted. I definitely think you're gonna cut a new path with your build.