Hi, for what it is worth I had an issue with the fuel gauge on my 1970 convertible. It was constantly showing empty and after trying various things including grounds and wire connectivity, I replaced the sender unit in my fuel tank.
My understanding is that ford gauges work off a variable resistance (approx between 0 - 78 ohms) from the sender unit. (~0 ohms = full & ~78 ohms = empty).
Once I replaced the sender unit, the gauge did start working but a full tank of gas sent the needle a fair way past the f mark on the gauge. This was annoying me (I also don't like picture frames hanging on walls at an angle) so I manually added a small amount of resistance (cut into the sender wire) until the gauge needle sat exactly on the F (full) mark.
This trick has worked well and it can assist in adjusting the gauge reading based on the sometimes pretty poor and inaccurate fuel sender units presently available.