I have observed (more than once) the inner lining of the fuel hose from tank to pump collapse and suck flat under vacuum, impossible to see it in operation.
Fuel injection rated hose is rated for pressure, not vacuum.

A light quick summary of the difference between "water pipe" threads and NPTF can be read here.

https://www.natool.com/national-pipe-taper-vs-national-pipe-taper-fuel/

(the tooling is different)

Pretty sure I have black plastic floats in all my carbs, so no chance of sinking.
If it was overflowing, it should be coming out the sight hole, which it is not.

I have had black plastic floats "sink"
very frustrating, when I disassembled to check it out they would dry out enough to float again for a day or two.
I had to weigh them on a gram scale to prove it.

SOHC Bob had one and together we studied it carefully, but never mapped it (obviously was not worth it)
The cylinder walls were massive thick. like 3/8- 7/16 thick, but they still had a large opening between them, maybe a 1/4" drill bit! It would have been a push to make it to a 390...

I have pinned a few blocks and many heads over the years. If there was metal missing I would head for a junkyard and pick another block to cut a piece from.  Not always the same engine even! There is one B-20 Datsun with a chunk of sbc pinned into it!  Fitting in a slightly larger piece made for a very good repair.

Note, if these are o-ring or mechanically sealed they may not be a tapered pipe thread at all.
They may be straight pipe and or dry-seal threads as well as a gasket or o-ring seal and they could be any thread pitch the manufacturer decided to use. Tapered dry seal threads are available in two tapers, 3/4" per foot" and 7/8" per foot (more common in automotive applications)
There are many variations of "pipe thread", rarely if ever are "water pipe threads" used in automotive applications, they have a built in "leak" at the root of the thread.
edit to add: gasket (mechanically) sealed usually indicates a straight thread.

C6AE, the short block came assembled from the machine shop. I hope he did them right.

I would want to know, it is not uncommon to find them installed (incorrectly) with the holes up, which is a clearance issue.

Not uncommon, For many years it was standard rebuilder procedure to sleeve this passage. I have done it three times,mall after the problem was discovered, once in situ, the others on the bench.
I used aftermarket small block chevy pushrod tubing 5/16" OD.  I drilled with a long drill bit 1/64 under then reamed with a long reamer 1/2 thousandth under the pushrod od. drove the pushrod into the hole and sealed it with  "wicking" locktite which is thinner than water.  All worked perfectly. What a mess to clean the system. The oil ruins the hoses, and the radiator has to go in the tank.
AERA had a service bulletin on this 30 years ago. You can find copies online. (I posted them to the old forum years ago)

https://www.ford-trucks.com/forums/1215888-75-ford-f250-390-oil-in-coolant.html#post16743222