If the manifold plugs have been in for a very long time its hard to
get them out .After removing them the manifold bolts have a 5/16
allen head . I have those manifolds on my boat and have had lots of problems
with them leaking,even with good mating surface. Just installed a
set of Remflex gaskets on mine because of leakage problems, only 2 hrs. on them
so I don't know how they will work.No other gasket has worked for me so far
even header gaskets. A lot of people use high temp silicone and no gasket ,but it
hasn't worked for me.