I don't think it would "look" like overkill, because it is a van door, if it was a small car door, I would say yes, try an 8"!
Seems to me, you have to select the speaker based upon what it's gonna be playing, midbass or sub bass, and also need to decide if it must be the same brand as the subs (for looks).
Not sure, but I think I read that Steve used a shallow mount RF sub in the Hoopty, and I have no idea which frequencies he is running to them.
Check the depth of that sub before you start, and get an eye for where the ring will set with the door closed, so you can see exactly how far it will stick out, then design for the 6.5"s around it.
I thought I wanted to try a pair of 8"s in my front doors, until I sat there measuring and visualizing them.