Perhaps it's not the front passages...
Even though they don't go anywhere, don't forget the two coolant ports in the back. (jamming a modded freeze plug in those back ports or epoxying (JB weld) them off wouldn't hurt)
On re-assembly, give the intake gaskets a good coat of "high tack". (the nasty red stuff from permatex) Make sure you let this stuff set up a day or two before putting coolant back in.
http://www.permatex.com/products/Aut...et_Sealant.htm
Have you checked the entire intake face with a straight edge to verify there is no warping? As asked previously, both covers?
Also some coolant dye might help here.