Can't get manifold off..

You should be good to go then. Don't go overboard on it, but use a good bead of rvt on the mounting surface and it should seal up without any issues. Just be sure of course to get both surfaces of the intake and valley very clean of residue, debri, greases, oils, antifreeze, etc. That will be what works against you if you don't. I always get a rag with lacquer thinner or acetone and wipe all the surfaces just before putting down the sealer. I also usually run some mild sandpaper over it before the lacquer. Glass smooth surfaces don't allow for bonding well, you need a little porosity to let it bite.