Doing the same thing right now. At the point of removing all the undercarriage stuffage. I plan to wire brush, wheel, degrease, and so on. I do not have significant rust problems, but will confirm obviously.
My plan is to first cover with Silver. Supposed to have additional filler in the Silver color. So it's not just a color change, but physical properties as well. Then I will come over top of that with Chasis Black top coat. Mine is a '66 Dearborn car, and I am of the belief that black was the color used in that plant. Other plants used a red oxide color. I am not a concours wealth of knowledge, so if color is important to you, verify elsewhere.
One of the details I have not yet sorted out, is what prep is required between coats. I have heard that you need to scuff the first coat. I have also heard that scuffing can be a PITA. However, I also heard that if the time between coats is short that scuffing is not required. I plan to confirm this with the POR guys via phone. I have also been looking at their "six pack" packaging. More expensive, but allows you to open up only what you plan on using, as the stuff has a short pot life.
BT