Thats crazy!  Remove the pop top off the other van, cut a hole in the 
roof of yours that matches the hole in the original pop top roof (its 
only about 4'x5').  Care must be taken to push all the wiring over to 
the side before cutting the new hole.  The hole needs to be finished 
with some sort of trim (sharp edges).  Then mount the new top.  I had 
this done on my van.....no welding required.  I am obvioulsy skipping 
some steps but you get the idea....

Chet
-- 
Chet Douglas - CRD Consulting - Peripheral FW,SW,and Drivers
(541) 754 - 1817