...............wait, isn't everything a challenge on a Z31? not familiar with them, but do the bolts go into the through the bellhousing and into the block? if so, one of the bolts might have broken through the water jacket....not good. I would drain the coolant, put JB Weld on the tip of that bolt, and tighten it back in if that's the case. that's the only way i can think to plug it without disassembling and taking block to machine shop......