I assume you wish to paint the exterior of your truck with POR-15
then paint over this?
They do make a tie coat which allows you to paint over the POR.
Personally I would recommend the Metal prep,although you could probably skip the Marine clean. Just make sure the metal is clean and grease/oil free.

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.