I'm thinking two or three different thin layers of the Right Stuff to seal the big gap and another layer of your favorite gasket maker for the last and final sealer up twocents
I use a thin blade screw driver to force the sealer deep into the space and let it set up over night before doing the second and then the third layer to make sure no pin holes any where in the gaps wrench scope

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)