i would highly recommend bill hirsch cast iron gray manifold paint.
I used it on my 340 manifolds 13 years ago and they still look like new.
I had very good results with the manifold paint from Eastwood cant remember name but it is a brush on product that covers very well and when the manifolds heat up the first time the coating cures very hard and durable. mine lasted for a good 2 years and was still holding up when I sold car.
Rust-Oleum stove and bbq paint - yes, really.
Its been on my headers for 2 or 3 years, only rust spots are where I scratched them or did not get good coverage-minimal. I even painted them first, then heated the one with a map gas torch to 'modify' it to clear the starter and the paint was fine.