So... better cooling of the center head bolt area isnt as good a fix? Seems Rex and Langer have had huge improvement in head gasket life by creating a water passage where there wasnt one, and directing a separate cooling line to it. Not knocking what your doin there Calvin. Perhaps doing both would be even better?
Fwiw there are lots of lessons to learn from the original 55-59 rev cool factory setup that could be applied to our modern stuff. Those gusher tubes directing coolant to valve guide areas is one of them. No room in modern heads for gusher tube...however their is certainly room to build an external water rail with probably 9 feeds just below the valve cover.(8 holes directed toward guides 1 hole toward center head area)
Keep in mind heat rises and cool sinks... its key to why early system works so well. I'll also add that most liquid cooled 2 strokes have water jackets around the cylinders yet there is NO return flow path outside of gravity, convection, and vibration! Water pumps thru head and water simply "falls" out of head into cylinder jackets. The 55-59 Pontiac only uses two 1/2" holes on block face for circulation to pump. Mainly its the bypass for when the thermostat is closed. Majority of water flow went thru heads via the gusher tube. Very little can actually "flow" thru the block.
Yep I've been studying closely!
Good explanation/pics of 55-59 rev cool and changes made in 1960 for conventional cooling can be found here...
http://www.pontiacsafari.com/EngineCooling/index.htm
My bet is, dry deck or not, we arent cooling as well/evenly as we could/should and that siamesed center ex port lets the center bolt expand/stretch much further than the rest.