The speeds you are describing with your '84 Westy is almost identical to my experience with my '84 Westy, also w/ 1.9, 4 sp.

I have no trouble cruising at 75 MPH, about 4100 rpm.
I recently crossed South Dakota with the bus cruising at 75-80 mph, using my Audiovox cruise control.
Of course I had to use the seat of my pants to help it up hills but then it is a vanagon! Sometimes I turn the a/c off going up hills too.

Going west into Cody Wyoming, I passed a much later Syncro that was obviously flat out also.  Perhaps the extra drive train offset the extra HP they are supposed to have.

Without a strong headwind, I can see 85 mph indicated quite easily.

Dont get excited yet!  I think your van is about normal.

Cheers,  Steve