I had a similar experience with the Performer RPM on a mild 440 (9:1, home ported heads, 509 MP cam advanced a few degrees, 750 Holley and headers), it worked great in my Charger with 4.30, 2.76, and 4.10 gears (I broke a bunch of rearends, lol) with a 4 speed.
That's a nice running Coronet...