Mitsubishi patented, in the early 80's, a counterbalancing shaft for
I-4's to eliminate the nasty second harmonic I-4s produce.  They charged a
fortune for the license and thus only Porsche ever paid them to use it
(944).  The patent is now expired and I believe many companies now use
it.  You are seeing much larger I-4's in the last couple of years:2.4-2.5
liters are getting common. I-4's are going to get larger, and better.
     The only modern V6 that fits, almost, into a Vanagon is the Audi/VW
V6. It is very doable but the engines are expensive and not available in
nearly large enough numbers to support 10 conversions a month.

    John
    Vanaru.com