The only time I had a valley pan do that was when I tried some collector insert mufflers and it made a ton of exhaust back pressure.

33 Plymouth Roadster - 383 - 5.90 1/8th 9.58 1/4
68 Dart - 340
66 Belvedere - 400

Windy Hollow Garage - https://www.youtube.com/@windyhollowgarage