Some SB's can have wet head bolt holes - they break into a water jacket. Pretty rare but some factory literature recommended sealant on the head bolts just as sealant is required on some manifold bolts. MA covered this in an article a couple of years ago. Of course it could be a cracked head or maybe gasket issue (maybe even the intake? not sure on a SB). Those are all the things I can think of that would put coolant under the VC.