Low speed clank when letting off gas

66 Dart with the 904. Ever since I installed the Lokar kickdown set up, I am getting a noticeable clank if I start from a stop, get to about 8 - 10mph, then let off gas all the way , the car shifts down with a hard clank. This only happens if I am drving slow, like in a parking lot. Seems like it is in 1st gear, i let off gas, and it shifts into 2nd automatically with a clunk.

Normal shifting is happening from 1st to 2nd at about 10mph btw.

any ideas>?