Aluminum 4 BBL manifold for a 64 question.

loubee,
Shouldn't be too hard to seal it, since the cooling system runs <16 psig, compared to ~60 psig for home water. I would try a penetrating seal. Drain and dry out the crack w/ heat, then try a high-temp super-glue (Loctite) or maybe "flowable silicone" (for windshields). Wrap the plug w/ teflon tape so you don't have to screw it tight to get a seal. Of course, everything is "simple" when giving advice from far away. Welding to cast aluminum is difficult, put sometimes possible.