Phil,
I feel this way about the conversion thing.
The VW Waterboxer, even rebuilt should last at least for 100,000 miles
plus with the proper maintenance.
The Subaru engine should do the same, except the timing belt has to be
changed  at least every 50,000 miles, and I am not sure what sort of
laborious task this may be with this engine buried in a Vanagon.
The engine that makes the most sense, if a guy was going to endeavor, an
engine swap, I think it would be the inline 4 banger, from VW.
Lots of room, to work on it, looks pretty straight forward simple to
convert, and from what I've seen and heard it causes the Vanagon 4x8
plywood air dam to fly.
I saw a 4 cyl conversion at an all VW show last summer, and the guy who
owned it claimed he shut it off at 115 mph. for fear of becoming
airborne.
And the engine compartment was a clean, open hole of engine---lots more
room in there than you would think.
Very impressive.

Later,

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Terry 85 GL