The green dart is a '72. It's got a 318 with Edelbrock air gap intake and Holley carb. I don't know anything about the engine internals a previous owner put it together. Also has a 727 transmission and 4:10 rear gear. It sounds nice and is street legal (lights/signals work). I've driven it...
My vote is valve body problem, not sure where but a valve must be in wrong. Study the shift kit install instructions and your book. Valve orientation, spring location and size as well as check ball locations are all critical. The good news is that the valve body can be removed without pulling...