[Dangerous Doug]

I had an "aha!" moment when I removed one of the thread reducer inserts from one of the holes---the inserts are capped on the inside end, making them a seal in and of themselves. So, to seal these holes you could just use four inserts, one on each end of both heads.

Looking for something else today down at the hardware store I found a separate section of the set screws and found the 5/8"-11's there.

So, in case someone else stumbles across this thread looking for answers...

DD