It was drilled very close to 0.750" dia.
I was going to just buy a steel "freeze" plug for that size.
But, I made some O-ring plugs on my lathe from aluminum stock to match thermal expansion of the hole in the head.
When I put them in, they are snug on the O-ring (compression), and I put a smear of RTV above the O-ring for secondary sealing and so it doesn't move.
It can't come out since the intake (RB spacers) basically cover all of it.

EDIT: Friend gave me some O-ring design info. A more typical design would be 0.082" depth (vs my 0.089") and more room for the O-ring width 0.143" (vs my 0.105").