Not sure how hot the manifolds (Stainless Marine) get but I'm wondering since there is water going thru them do you need to use a high heat paint?

They should be alright with just a good scuffing or if bare metal some zinc chromate primer and a good two part epoxy if you want it to last.They should never get over 190 degrees.You can skimp and hit em with some cheap enamel too,probaly hold up just as well,shine just won't last.