Slightly Embarrassing Question

1971 Dart. 408 (360 LA block). I’ve been chasing what I think is a drivetrain vibration for a couple years (!). Lately, it occurred to me that maybe it’s an engine balance thing. I don’t think it is... the vibration is in the seat of my pants, starts being noticeable at 30 mph and gets steadily worse the faster the car moves. At 80 it’s shaking pretty bad. Also, at 80, if I mash in the clutch the vibration changes little if at all. However, in that condition, if I rev the engine to, say, 3000 rpm, I can definitely feel a “lighter” vibration, that I associate strictly with the motor revolutions. Four speed; Detroit locker diff, 3.55, centerforce clutch. Thoughts? If my engine balance was off, I would definitely know that, right? I don’t know if my configuration is external or internal balances, but my builder (I won’t mention any names yet because I think there’s no problem there) says it’s internal balance and a neutral h balancer is what I need, and that’s what’s on there currently. Any and all responses appreciated.