Here's a picture...I realize it's not the best, but it should help. The long adapter shaft runs down into the head and then the distributor bolts to the head. If the car backfires when you try to start it, the distributor is 180 degrees out. The distributor only mates to the adapter one way because of an offset tab on the bottom of the distributor. Reinstall it and try again. Note in the picture where the vacuum advance is...use it as a guide when lining up your distributor.