underside paint choice

for me, it would depend on how I wanted to use the vehicle. If it's going to be a show car, I think that Mopars look the best with the undercarriage painted to match the top side. If it's going to be (like my '68 Charger project) a daily driver that will sometimes see inclimate weather and gravel I'd use a coating such as POR-15 top coated with undercoating to prevent rust, help insulate, and quiet the ride. Our Charger was completely stripped, with new floors, trunk, quarters, etc... The undercarriage was stripped of all previous undercoat and rust. I even sanded and rustproofed the inside of the frame rails where ever I could reach. Our goal was to have a very quiet (from road noise) car that could be reliably exposed to the elements without being overly concerned about rusting. Even though we have everything new and could focus on detailing the bottom of the car, we're still going to undercoat. We spent over $400 putting POR-15 inside the doors, quarters, roof, ... - everywhere. Time will tell if it was worth the effort.