>Funny, I had a 75 1800 Bay which had a slip-in 1916 kit. Otherwise it was
>stock. It cruised quietly and happily at 130kmh (81mph), as per the owner's
>handbook; on the flat it would hit 140kmh and still be accelerating

Two reasons I can think right off the bat:
1.  My vehicle is heavier than yours. The Vanagon is heavier than a bay
window bus, the Westfalia even more so.
2.  Westfalia has a larger frontal area.
3.  Euro-spec motors, obviously not the one in my Westy, are more powerful.

My first experience in a Vanagon was in Austria where I and 8 other people
with their luggage topped out at 145 km/h.  This was an air-cooled '82
9-passenger van in 1982.  Obviously the van had more than the 67 hp at which
U.S. models are rated.

It sounds like you should have held on to your '75.  What a ride!