I have a '59 dodge coronet D500 with a freshly rebuilt 383 and the original transmission that was rebuilt by the previous owner 25+ years  (and 20k miles) ago. 

The car has been off the road for several years and I've gotten it back together, but the transmission shifts hard into reverse with a bang and will break the rear wheels free (chirp!)  even with my foot on the brake. The transmission was doing some other weird things until I adjusted the Kick down lever all the way forward to eliminate that is a possibility. Naturally I won't downshift now but that's the least of my worries. 

I suspect the previous owner added some sort of a shift kit a part of the rebuild, but I don't know if that would be the culprit. Could this be a pressure or band adjustment issue? I've never monkeying with those sort of things on this transmission. The only thing I've done was drain and change the fluid, And based on the recommendation of the transmission rebuild or who inspected it, I used Dex/Merc with 1 quart of type F. I'm wondering if the added friction of that portion of type F is causing the issue.