Why not put the oil cooler in the path of the air flowing in the drivers
side rear air intake (the big black vents near the back of the van)?
This would seem like a good place to me, in my watercooled 87 this vent
seems only there to pass air into the engine compartment, I wouldn't
think the added heat of an oil cooler would change things much on the
watercooled models.  Just a thought, I personally don't need an extra
oil cooler, mine runs just below the LED to slightly above depending on
load and ambient temp.

dave