I have had no problem with the stock 415 chain hanging on my drive sprocket. I would think that this is the same sprocket as on your box. I think that the chainline is critical and could cause your problem if it's not correct. Using a number 41 chain would give more clearance if it is a thick sprocket. Ref: Azusa Engineering, Inc. Online Catalog - Roller Chain, Motorcycles, etc.
Were you able to do much riding before your problems? If so, did you find that the box was very noisy? The old boxes with the straight cut gears are pretty noisy, but I am enjoying the smooth clutch action, good cruise speed and good hill climbing ability using a 36T sprocket.