I just scuffed the exhaust ports and crossover with emory cloth, wiped with brake cleaner and sprayed 'em with a couple of coats of VHT cast iron Hi-Temp paint, then top coated with the standard Dupli-Color Pontiac Metallic Blue paint. I probably spent 10 minutes max on prep and paint.
The blue turned silver but I've had no peeling or rust through since I painted the engine and that was like four or five years and 6000 or so miles ago.
I think the secret is the high temp paint undercoat.