The stock system use 2 level sensors. One in each saddle of the stock tank.
Is that for all models/engines? I don't have direct experience, but reading the wiring diagram that was posted, the "Fuel Level Input 2" circuit goes through connectors (C434) that are
only labelled with "5.4L" while the "Fuel Level Input 1" circuit connectors are labelled for "5.4L" and "3.7L / 5.0L". So in my reading of the diagram, only the 5.4L-engined cars use 2 fuel level senders. That's if the wiring diagram is correct for the OP's car.
To the OP, if my above interpretation is correct, then you need the Yellow wire with a Violet stripe, and the Green wire with a Blue stripe. They should be on pins 15 and 16 of the instrument cluster connector, and pins 1 and 2 on connector 433 at the fuel pump.
Looking at the instructions for the MeterMatch, it assumes the fuel level sender is grounded, which the Mustang sender in the diagram is not. Since you're using a fuel cell with likely a non-stock sender, you'll need to wire your sender to the MeterMatch appropriately. Make sure the "ground" connection on the MeterMatch goes to a good ground.
If I'm wrong and you have 2 fuel senders, I
think you might still be able to fake the signal with the MeterMatch. You join the Yellow wire w/ Violet stripe at pin 15 of the cluster and the Green w/ Orange stripe at pin 18, and use both those as the "+12v" input into the MeterMatch. Then take the "to gauge" output of the MeterMatch and connect that to both the Green w/ Blue stripe at pin 16 of the cluster and the White w/ Violet stripe at pin 19. Program the MeterMatch as in the instructions.
Good luck. I'll cross my fingers for you.