I have the same gear in my 1970 Mk2 - 2-speed motor.
It doesn't oversweep.

Van or sedan? Just curious if that makes a difference.

Sedan: Mk2 S police spec so 2-speed wipers were standard. It's the original wiper motor for the car, just with new contact plate and self-park contact leads.

To narrow down the situation, what year is the van and do the wipers park on the left or right (i.e. is the scuttle flat or the later indented version)?

Your scuttle panel is for right hand parking wipers using the DR3 wiper motor. The motor you are trying to use was introduced for left hand parking wipers. It can be made to work though.

Gambo - I think that makes a big difference. I don't have the knowledge to assist, but plenty on here will have done it previously. Bill's car has the later indented scuttle and wipers parking to the right, like my car. I think the toothed plate has to be changed to stop the sweep in the correct place.

Hopefully someone will wander past here soon and give the precise steps to fix it.

Last thing I want to do is post uninformed comments, but perhaps this discussion is relevant while waiting for advice from someone who really knows. It was for a 12AUW wiper motor in an earlier right park De Luxe like yours.

https://ausmini.com/forums/viewtopic.php?f=2&t=96518