Old school underhood superchargers...anyone remember?

Paxton style (centrifugal) doesn't make constant boost as rumble mentioned but they are a lot more efficient as far as heating up the intake air and sapping power from the engine (they use a lot less than a Roots). If I was going to the trouble I'd use a Lysholm screw compressor like on newer cars they're constant-boost but still more efficient than Roots (which is a 19th-century design btw), but still a tad under centrifugal blowers. I built my 5.9 Magnum in my Duster with head studs and Cometic head gaskets so I'm tempted to put a Procharger (centrifugal) setup on it eventually. Those you can pretty much install and remove without modifying anything major like manifolds or belts. After I add EFI of course.