Yeah I would not want to introduce water into there. To ans your Q accessing the block dist hole would let me checkout the (2) rearmost cooling passages between head/intake. A shop here has a smoke machine that pumps it in to 5 psi he said & it'd be 1/2 hr shop time ($40). I might just bite the bullet & replace both head/intake gaskets & if that dont do it then the tcover gasket.