Well.. is it a stock externally balanced 360 (LA) or magnum 360? They're balanced differently. Could be harmonic balancer as well. The more info you have the easier to figure the culprit. Also if you have the 904 the 10239 is meant to use the stock 318 converter which is neutral balance - no weights.
It had a 727 in it to begin with but it was slipping so I found a good 904 with torque converter, the converter does have some small weights on the side just behind the ring gear and it's an L.A. 360