1986 2.1 in a Westy here.  I try not to go over 4000 rpms for
cruising.  The engine definitely seems comfortable going even faster.  But
as others have said, the smoothness/quietness of the wbx way back in the
back of the vehicle is deceiving.  I have the auto trans, so 4000 rpms
seems to vary between 65-70mph indicated on speedo.

my ex-SO had a 1977 rabbit, and in the owners manual it listed something
like maximum speed: 103mph, maximum cruising speed: 103mph.  I bet there is
some secret VW info somewhere about what the engine(s) did in dyno testing
at different loads/rpms.  I'm sure these vans got driven pretty hard in
their homeland on der autobahn too.

Edward

ps. I think 4000rpms in my Golf Tdi is 110mph...