Garett,
   It sounds like you have been bitten by the Jaguar bug and you are
joining the rest of us in an attempt to bring your '77 Jag up to speed.
Congratulations!!
   What you need to do with the petrol tank is determine,   first off --
how bad the leak on the tank actually is. If it is a pin-hole or two than
you can easily repair the tank and it will last for years.  If you are
talking a split seam or a damaged weld than the problems are a bit more
intense.
   In any case, I have found that keeping the original configuration is
usually the best way to go.
   On my 1967, I had a similar problem with my RH tank (it always seems to
be the RH side for some reason!!).  I had a couple of pin-hole leaks as well
as a TON of flaking rust inside the tank to say nothing of a plugged line
that drafted the petrol from the bottom of the tank to the external pump.
   I took my tank to a local radiator shop and had it "Boiled"   What this
means is that they immerse the tank in a vat of phosphoric acid and it
essentially eats up most of the rust.  Once this was done, I was able to see
the exact extent of the damage.  Since the holes were only pin-hole and the
over-all integrity of the tank was fine, I coated the inside of the tank
with a tank sealant coating made by POR-15.  www.por-15.com   This coating
is designed to deal with tanks in the condition you describe.  I also coated
the outside of the tank as well with this sealant just to make sure I had
covered all the bases.  I then painted the outside of the tank with an epoxy
based paint.
   My '67 already has an external pumps so I did not face the pump problem
you have.  My only thought is that it sounds as if your problems are more
with the tank than the pump/sending unit.  Once you repair the tank, you
would most likely be better off with the same pump/sender configuration.
After all, the original worked pretty well for 25-28 years.  If you replace
them and they work for another 25 years -- how can you complain?
  I understand that the cut-over switch is the major problem on the XJ6
although I have no idea if yours was built that way.  I believe the later
models used two tanks, but one pump so you may be OK.  I like the idea of
two separate pumps in that if one decides to go on strike, you always have a
back-up.

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