Side-oiler mystery problem

[HSSS427]

I will first pre-face this thread by saying I'm not interested in mentioning names or finger pointing; just looking to leverage the collective knowledge for ideas and help.

I have a 427 side-oiler, C5AE-H block, with Shelby American Aluminum heads that have been mildly ported, and Edelbrock Performer RPM intake. Engine was freshly re-built 400 miles ago by a very reputable FE engine builder.

I ran the car hard one afternoon for about 90 minutes and then parked it. No noticable performance problems or missing. Next morning I pulled the #1-#4 plugs to get a reading from them. I noted the #2 plug was very white and clean. Given past history I feared it was from water in the cylinder. I turned the crank by hand and sure enough water shot out of #2 spark plug hole. It was a fair amount of water appearing to be enough to fill the cylinder.

I then removed the intake and valve train and filled the water jackets on the head as high as possible to see if there were any obvious leaks into the intake or exhaust ports on the head. Nothing obvious spoted.

I then ran a leak down test on #2 with cylinder head at TDC - leak down passed. I then ran it again, with the cylinder head at bottom of the stroke and got air bubbles appearing in the water jacket openings.

After discussion and debate, I pulled the engine and took it to a very reputable local FE shop to get repaired. They pressure checked the heads and the block with the engine still assembled and both passed.

Then decided to dis-assemble the short block, clean and magnaflux, do all the surface checks, etc and further pressure test the water jacket with 75 PSI. Still no indicator of leaks or issues. Only speculation at this point is potential for leaks via the head bolt holes via cracks in the bottom of those hole castings that protrude into the water jacket.

Next step is to re-assemble, swap the heads to opposite sides, use sealant on head bolts, and ensure good head and intake gasket seal, then put it on the dyno and see what happens under some hard runs.

So, am just hoping that maybe someone has seen this mystery before or has a tip that might help explain it. If I try to just think of the basic paths water could take to enter the block, and the fact that the block and heads appear ok, then I'm lead back to the head gaskets, with perhaps a nuance as follows. It has something to do with aluminum heads on an iron block and when it gets hot, there is some type of expansion that causes the gasket to leak. Thus, the water leaks in when the block and head are hottest ie after shutting off the engine when it's heat sinking with no coolant running through the engine. When it's cool, it all seals back up.

Appreciate your help and thoughts in the great side-oiler mystery!
Thanks!
Scott