Woody, Valance ?, You mean the back of the cab. Are the doors on the truck and your going to paint the jams?
I know what your up against, to paint everything at the same time means too much overspray usually.
Then there's all that masking to paint just parts which I have done to achieve the result I'm after.
The only thing that comes into play using a metallic is the application.
You just have to try and duplicate the sane way of applying the paint every time. A little wetter, a little dryer will render a different look.
I think I would be using your approach to situation because you don't want to have to deal with what the over spray will ruin.
Dan