Not sure if you're aware of this so I thought I'd better just mention it: Fuel quantity senders and gauges need to be operating in a compatible resistance (ohms) range in order for the gauge to read correctly. Not all gauges and senders are compatible with each other. So the Ford factory sender referenced by ND may or may not be compatible with your non-Ford factory Sun Pro gauge. You need to know the ohm range the gauge is operating in and the ohm range the sender is operating in. They need to be the same. Unfortunately, industry wide, there are a lot of differences. This is irrespective of any issues you may or may not have with the IVR (Instrument Voltage Regulator). The test you ran, grounding out the sender wire to see if the gauge has life and full travel gives you that answer only. It has no bearing on compatibility. Hope this helps.
'66 F100 LWB, CC, restomod, roller 390, NV4500 5 spd,
'66 Mustang conv, restomod, 5 spd, 302, 4 wheel DB,
'64 Falcon Sprint restomod, 5.0 SEFI, 5 spd, disc brakes
'63.5 Galaxie 500XL, 427-8V, 4 spd, 4 wheel disc brakes