You have to be INSANE to have long-tube headers on a big-block A-body Mopar!

A big block in a Dart is already a fast car with manifolds. The 67 and up cars accept the B or C body HP manifolds with little to no mods and flow very well. Unless you're building a really hot street or race car, it's just not worth the trouble. I've seen a 71 Dart with a stock stroke 383 get into the mid 11s with manifolds. That should be plenty "fun enough" for a street car, "you'd think".