Since the Holley SD was brought up, I'll comment that my personal experience with it on a magnum spec 383 in a heavy car was very poor. I used it cuz the a/c brackets fit it, but I lost a lot of tq. No empirical test, but same car, same engine, same carb, same tires, same stretch of road: with stock intake and 3.23 gears it would light them up from an idle. With the SD and 3.91's it would only squawk them from an idle.
The artical linked shows it's down on tq at 3000rpm, from my own testing from idle it's waaaaay down. perf rpm and efi gave a better response than either above.
