ya know bill you could be just like totally awesome and sell him the stage 3 heads you just got in, HAHAHA i know im funny huh???
the coolant tank should be about 1/2 full when warmed up... i think... its been so long since i looked at mine hahaha its got fluid so im ok with it
the LIM is the Lower Intake Manifold... more importantly the LIM Gasket is what causes problems in the late 90'* models, not too often but it does happen, causes similar problems to your situation and a blown head gasket effect but its in fact the lim... and its recommended to go with the better Metal/Rubber gaskets vs the original gaskets... dan and bill both should be able to direct you where to go should you need a set.
Part# 89017816 - 68.96+tax from GM... not sure if thats the original or the metal upgrade but thats dealer # and price