Sean -

Be sure when using those O-Rings that you DO NOT insulate the sender from the
plate.  Why?  Because the plate is the grounding point for the sender.  On my
80 Vanagon with the same identical setup and I used an extra copper washer on
the inside to prevent leaking.  Without proper grounding of the sender one of
two things may happen 1) A false or intermittent gauge reading, or 2) No
reading on the gauge at all.

Just my .00000002 cents worth

Jim / BusBoys@aol.com