Help! Sable needs fuel pump, but don't know which one

I own a 1988 Sable LS, 3.0 FI. The fuel pump pressure at the rail is 15 PSI, should be between 35-45, per Chiltons manual; I've already replace the fuel filter and pressure regulator. This particular model
year came with two different fuel tanks, 16 and 18.6 Gals, which of course use two different fuel pump part numbers. I would like to purchase the new pump PRIOR to dropping the tank, so I can simply remove the old pump, reinstall the new pump, reattach the tank, and be done with it. The problem is, I do not know how to tell, short of filling the tank, (which cannot happen, because the vehicle doesn't run well enough to drive, also I have no desire to drain an ENTIRE fuel tank) which tank is installed in this vehicle. The vehicle came with almost every option offered for 1988; Digital Dash, Power Seats, Door Locks, Power Antenna, Climate Control, etc. etc. So my question is, does anyone know how I can tell which tank this vehicle has installed? I called the local Mercury dealer, they offered little help, aside from suggesting I locate a stamped number on the tank, which may identify the size, but I don't see this, maybe I'm looking in the wrong place? Any suggestions would be appreciated, thanks in advance... (BTW, the prior owner is deceased, no help there.
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Jay wrote:

Unless there are some differences from the Taurus, the pump can be had independently from the whole pick-up assy and gage sending unit. You should be able to just purchase the pump. That's what I did. Local parts store for about $65 5 yrs ago.
With that said, I would caution about just replacing the pump because the sending unit is of cheap design(pc board with silk screened traces that wiper arm moves over) and will fail and this item doesn't seem to be available by itself. My sending unit failed within 6 months of replacing just the fuel pump.
Mickey
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