Looks like you've got some cool hardware on that BBC. The exhaust guides actually pass through the water jacket in the head. If it moved for whatever reason (valve seized, bad fuel, bad installation, etc) it could cause the leak. A pressure test at a competent machine shop will tell you for sure.
(I know I'm a N00b here but I manage a 10,000 sq ft automotive machine shop. Not saying I'm a guru but I have a bit of engine experience I'm happy to share

).
BTW, warp spec on those heads it .004" overall. On higher performance engine if I can get any feeler gauge under a straight edge anywhere on the head I recommend a shave.
Good luck with the project! Looks like a blast!