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"... BUT maintaining the pressure on the cylinder and spraying oil on the head studs and nuts, bubbling of air escaping at random studs at the threaded end as far back as #6 cylinder was observed..."

Remove the studs around #1 cylinder (and preferably ALL studs) and put a small amount of NON-hardening permatex or other suitable such sealer on the stud threads that screw into the BLOCK. This is standard engine overhaul procedure for all water cooled engines.

I second Keith's suggestion. If there is NO crack, then somehow it must be leaking into the water jacket from #1 head bolts. I would put sealant on all the studs. I also like Eric's copper coat suggestion for the gasket. I used that on some intake gaskets and was able to reuse them (long time ago). Never had a leak at the intake.
Did you try to pressurize #1 with the piston at the bottom of the stroke? May get more/quicker pressure build-up.

If the head was milled or ground then use a micrometer and spot check a few of the head bolt bosses on the TOP of the head to see if they are parallel with the block mating surface of the head. Is there evidence of broken head stud washers???

Thanks for all the brainstorming, guys. To answer a few of your questions, all the head gaskets installations have been done with Permatex copperized spray, I've used this stuff forever on L-head head gaskets and am a firm believer. Also, on one of the many cylinder head reinstallations, all the washers were replaced with aircraft-quality machined flat & parallel washers.

At this point the next step would appear to be the removal and reinstallation and/or replacement of the cylinder head studs with sealant, though assuming the head gasket is holding, it's difficult to see how combustion pressure is reaching the studs. I've seen coolant come up around a stud but that doesn't give a passageway to the combustion chamber if the gasket is sealing.

I've also considered a crack in the block below the exhaust valve seat, but this would give rise to coolant in the exhaust but shouldn't allow combustion pressure into the coolant because of the absence of signicant pressure in the exhaust manifold, and of course we still have a leak with the valve closed.

"all the washers were replaced with aircraft-quality machined flat & parallel washers."

Are the washers HARDENED??? They can not be soft.

Just be sure that the head bolt boss at the top of the ehad is parallel to the mating suface of the head. Most auto machine shops do a sloppy job of resurfacing heads and in the process the resurfaced area may be at an angle to the original sufaces of the head stud bosses at the top of the head. My gues is that + or - .005 inch variation from one side-to-side of the boss would be ok. Maybe check an NOS or other know good head to see.

Yes, the washers were hardened. No, we didn't examine the bosses with a micrometer, but remember that the problem began years and many miles after the head was resurfaced.

I'm really still at a total loss on this problem. A crack in the block explains everything, but it's not to be found. Please guys, keep thinking about it.

Mite be a good idea to make an air hose addapter to fit the stud holes in the block after the studs are removed. Try to determine the offending holes and why they're leaking. Regardless, the permatex should seal the threads just fine.

It would be good to know if and why the stud holes suddenly started leaking.

Good points and thanks for continuing to think about it, but there really is no way for the combustion gases to get to the stud holes unless the gasket fails (no sign of that) or there is a crack (not yet found).

The inescapable conclusion I continue to reach is that there's a crack that has thus far defied finding.

Have you sprayed the deck with that crack detecting dye?

Also do the straight edge check on the deck to make sure it completely even and no warpage. It's a quick test!