Small Valve AL Heads

I got 1.865" centerline distance between valves on the 675 heads. (Measured outside to outside across 4 valve pairs on one head, averaged, and subtracted .375" stem diameter.) With 1.78" and 1.6" valves, that works out to 3.535" outside edge to outside edge on the valve heads..... juuuuust will fit into a 273 bore LOL!

I am beginning to think this is just old 318-Poly design dimensions that were fudged onto the 273.

I am also starting to think that some like the Edelbrock Performer head will have no more than about 3.8" outside-to-outside edge on the valves. This is because it is advertised to go on a 318 block, plus studying detailed pix on their website.