Been there, done that on an older Bus and on a vanagon. 
I have also responded directly to several members of the list regarding this
question.

In short:
There are several types of poptop roofs available which have different
mounting needs.
The Westfalia roof can be mounted by cutting out the middle segment of the
vanagon roof and leaving all the frame intact. The bed has to be constructed
from plywood by adapting a frame to the roof curve and making the front part
a slider. (The Westfalia roof has a fold down front part.) The advantages
are that you can make the bed a little longer and that there is more
headspace for the rear passengers and a rear top cabinet. The disadvantages
are the loss of space up top which means that the front protion of the
cushions does not fit under the roof when it is folded down. The job
requires mostly woodwork and no welding.

Optional:
I cut out a portion of the front roof segment (above driver), cut out the
"bucket" in the Westfalia luggage rack and fiberglassed a sunroof into it.

Now you can have fun.

Bj�rn

Bj�rn Ratjen

Mill Bay, B.C. 

1987 Syncro Est Alia (self camperized)
1995 Passat TD
(1982 Vanagon Diesel)
1979 VW Bus (self- camperized)