The '72 Westfalia I used to have appeared to have been made from a 
  sunroof bus with no sunroof. I used to stick skis and poles in the cavity 
  into which the sunroof would have retracted on snow trips. The pop top on
  the '72 was the one that pivots in the front and has the luggage rack
  at the rear of the bus.

  My '74 Westfalia has the pop-top that pivots at the rear. The luggage rack
  is a "tub" up front. Much of the top of the bus is cut out. The opening
  in the roof extends close to the rear of the bus. HOWEVER, there IS a welded
  in reinforcment across the width of the opening at about the midway point,
  just behind the sliding door opening. I'd be nervous about converting 
  a non-pop-top bus to a pop-top by simply cutting a big hole in the roof. I
  suspect that the reinforcment beam was placed there out of necessity. 

  I'm assuming that all the '74 and later Westies with the rear hinged pop-top
  are the same way. 
 
  An oddity: I spent several hours in Jack Stafford's '73 Westy during the
  Red Rock expedition to the Burro Schmitt tunnel. In '73 the pop-top still
  pivoted at the front and there is a cot instead of a bed up there. His 
  '73 was NOT made from a sunroof bus. Both the roof opening <larger> and
  the cot mechanism were different then my old '72. One year only arrangement??

  It may be possible to put a pre-'73 pop-top on a bus that already has a 
  sunroof.<?>

  Dave