If the holes are exactly 3/4 inch then pick up a couple of Lawson expansion plugs and drive them in there just like you would on the end of factory heads.
https://www.lawsonproducts.com/lawson/Steel-Expansion-Plug-Regular-34/3085.lp

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").

Nice fix

I would have been tempted to try a brass freeze plug driven with interference fit
Good luck on this

That should work. Do you have a pressure checker for the coolant system? I have a little hand operated pressure checker that pumps the coolant system up to 10 or 15 lbs. It is handy for finding leaks before I leave the garage.

Well I have had a B1 or 12 and I have NEVER done it. We do pull water from the exhaust side between the siamesed exhaust ports but not on the intake side. Easy fix plug the whole guess I never paid enough attention to the earlier pics of the heads.

As a self-proclaimed O-Ring junkie, I LOVE IT!

Nice solution. 👍

Note to Moparts.com readers:
A friend of mine emailed me some actual design criteria for O-ring grooves. (Thank you)
My part is a little on the tight side for width, but it will be fine.
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").

Parker has o-ring design manuals that you can download. I have one taped to the wall by my CAD station since I use it all the time.

Crap.
Setback today. I broke the drive hex on the oil pump drive.
I have primed this motor with a drill at least a dozen times during different phase of the build (no lifters, with lifters but no pushrods, pushrods no rockers, and fully assembled.
I have run the engine in the garage about 4 times before today. But when I started it today it had zero oil pressure.

I will have to pull the oil pump and do some investigating to see if anything else has a problem.

Bronze teeth look ok, maybe debris got in the pump? Hard to believe, your build looks really sanitary.

Well that's not good.............

That is no good but at least it broke in your garage rather than at 160 mph in the lights.

I've had that happen more than once Look inside the oil pump to see what locked it up

Well that stinks, but as pointed out couldn't be a better time for it. Time to see what locked it up.

I unbolted the rack and pinion so I could remove the steering shaft and get the oil pump out without removing the engine.
The hex tip of the oil pump drive was still in the oil pump, as expected. There was no debris in the oil pump.
The pump turns smoothly by hand (with a hex rod inserted).
I measured the outer gear to body clearance and it was fine at 0.009" (brass feeler gage). Spec should be less than 0.014"
The tip clearance between inner and outer rotors is within spec (less than 0.010"). It was less than 0.006"

Could an engine backfire cause this with stiff valve springs? The day it happened I tried using the MSD start ignition retard, and the engine didn't like it. But then it started the next try (and no oil pressure).

I would never say never on the backfire, but can't imagine how that could occur, but again never say never. I personally hate start retards they seem to cause more issues than they solve. With a separate start and ignition circuit, like 99% of racecars have I have never seen the need for them. I would lean more toward a defect in the oil pump shaft itself, that shaft sure does not look like it was "twisted" off at all. Curious what weight oil and ambient temp?? Maybe a slight alignment problem, I have seen it before and on a older version of a World block......But you would likely notice that on install.

I don't normally use the start retard. I just decided to try it.
I do have the separate starter and ignition switches, which is how I normally start the car.
I have never broke one of these oil pump drives before. But I only ran a roller cam 1 season. That was my 540 KB, 440-1 head motor; which sold that fall.

I am using 10W-30 oil, and it was just after lunch, so the ambient was about 70 degrees.
The previous day I set the ignition timing (32 deg) and valve lash HOT. Just before lunch I verified the COLD 0.006" lash.
I will try to get a better picture of the break with my Olympus camera instead of my phone.
But it is, what it is. I just need to check around for any other signs of a problem.

During an early mock up of the block, I did insert this bronze gear drive in the block to check it turned smoothly in the block bushing.

I broke one one time, It was because of the face/gear to cover clearance. It was one of the old dc extra thick gears with spacer. It was obvious what the problem was though. Yours doesn't look bad. Just throwing that out there.