"I thought I'd pass on some info.  Our KC Kombis NEATO Chapter had a tech 
session yesterday on the installation of windshield rubber.  The 
candidate for this project had new rubbers installed a few yrs back, but 
they were not working out real well--ill fitting and leaking *with* an added 
black silicone bandaid fix. To make a long story short here were the 
solutions:

 * New Rubber from Vintage Parts, Inc.
 * The use of an original style 'thin' replacement glass instead of the 
     thicker custom made US spec safety glass
 * Redrilling or augering out the original drain holes in the base of the 
     window sill
 * Sanding out the surface rust and lightly treating the area with 
     Rustmort or POR 50 (or whatever)
------

I like to smear some roofing tar on the window ledge before putting in the
window, and after doing any rust & paintwork neccessary.  i dont trust any of
the window seals that dont havethe original contour (which means anything not
NOS)    Lovely thing about tar is that when(if?) it gets warm it squidges
around and reseals.  Also prevents more rust under the rubber.   Havent done
any long term studies, so cant tell you how it'll be ten years after.

Clara