the 86-91 passenger van a/c system doesnt need the center ducts or front duct
to be functional.  in fact 88-91 has the a/c controls in the dash, so the
only thing in the ceiling is ducting.   without the ducting, air will still blow
out the hole in the rear..as blowers and evap are all contained in the rear
unit.    so if you are doing a poptop conversion to a 86-91 pass van, and still
want functional a/c, you dont have to worry about anything.  the only thing
you lose is the ability for cold air to blow in your face, which i found
annoying anyway.
 westys with a/c just blow air out the overhead rear anyway, so it wouldnt be
any diffrent from that.

chris