Need Help w/ Intake Details for '67 920 Heads on '66 273

I've been struggling to find a proper after-market intake for my 920 heads (5/16" bolt holes as measured w/ caliper). Is this one-year head an unloved bastard? If I have to modify an intake or the bolt holes in the head I'll need some details.

I measured the bolt size by using a caliper on the inside of the hole, inside the threds. Do I have to measure the outside of the threads and will that be the 3/8" bolts?

Thanks for your help.