Ken, I used POR-20
To be fair I haven't driven my truck in anger yet,so I don't know how it'll hold up.
I do however run my engine once or twice a week for 15/20 mins or so, just to keep things loose.
So far it's still looking good,it hasn't peeled,flaked or discoloured.
I got the headers blasted first to provide a good key for the paint.
I'm planning a third coat before it hits the road.

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.