I don't want to rain on your parade, but it's very common with a large backfire to possibly damage your mufflers and or resonators if you have them. to check first visually inspect the body for any distortion or a bulged puffed out look. The next thing to check if to see if any of the welds or crimps let go on the inside for the baffles. This is easily accomplished by smacking the main body repeatedly hard with the side of your fist and listening for internal rattles. When a baffle is loose you end up with a weird rattle/buzzing sound that usually only happens at certain rpms.Hopefully yours is well. And I don't blame you, who wouldn't be attached to a low production H.O. 4 speed car.