With that last e-mail, other insights came quickly to mind.  If it is true
that the GM cores have blocking plates to create a serpentine flow path, and
since it is true that these cores DO have the inlet and exit tube on same
"end cap", i.e., both on the "top" as installed in the car, with tubes
running up and down.
   1)  They might be mounted "sideways" in some planes, with tubes running
across, and unmodified fittings being on same end, one high, one low.

Given that there is serpentine flow, with coolant inlet and exit pipes on
same "end", then it may be either a 2 pass or 4 pass serpentine path.
   -  A two pass core would have a blocking plate half way down the tubes
from inlet, with fluid going across first half of the tubes, and coming out
the next half and exiting out the "end" (top in car) without any internal
tubes (such as what Steve removed) to facilitate that flow path.
   -  A four pass core would have a blocking plate 1/4 way down tubes from
inlet, 1/2 way down on opposite end of tubes, and 3/4 way down from inlet on
side with inlet and exit tubes.
   . . . . . --  I can no longer understand what the "removed tube" was
for, unless it was the "exit tube" of a "3 pass" serpentine flow, i.e., it
was welded to the outside wall of end containing both inlet & exit, and its
purpose was to "reach" across to the other "end cap" (go across parallel to
the tubes) and provide a return path for fluid that passes across the bottom
1/3 of tubes, so as to allow their flow to come back to the exit side.

Anyway, given any number of serpentine passes, the fact that there are
blocking plates is the primary cause of "hard to purge air out of cores".
I've been assuming that there was no serpentine flow, that all the fluid
came in one side and out the other like a radiator.  This wasn't even a
logical assumption, given that when I looked at the cores still in the car,
both tubes were on the top.  That is already different from a car radiator,
where the water comes in top right and goes out bottom left.
   -  The fact that there HAVE to be blocking plates in there for both the
inlet and exit to be on same end cap gives us an explanation about
difficulty of purging air.

Now, depending on how each builder might choose to modify his cores to weld
on hose fittings, if he was unaware of the blocking plates - and where they
are physically located inside - then a builder might chose a location to
weld on a fitting on the opposite side (unlike the way it was built) so that
he might be "short circuiting" the serpentine flow path so he is not getting
flow through part of the tubes.
   - How many have felt their cores to assure all tubes are equally hot?
Steve may have really opened up this thing to a new level of understanding.

Finally, if we can find out exactly where these blocking plates are, then it
might be worthwhile to install pet cocks (bleeder valves) in those parts of
end caps where air might be trapped - open bleeder valves during filling
until liquid comes out then close.  Given that consideration, then "how the
cores are oriented" (upright like in car or on one side or the other - 3
choices) might lead us to ways of minimizing the amount of trapped air that
has to be purged.

David