What is "correct" for undercar undercoating?

I am close to the point where I will have The Duke's underside completely rust-free and coated with Rust-Oleum Rust Converter. This car had some sort of tar undercoating over the paint underneath (poorly and spottedly applied) and I want to stick to close-to-original. I was going to paint the underside with the body color paint of choice, but If I'm undercoating over it, who gives a rat's ***? You won't see the color anyway!
Anyway, did Chrysler originally undercoat the entire body, and if so, what is recommended as a spray-on? Westley's, or something else?

Oh, sorry.....'72 Plymouth Duster