If I recall correctly (and with me you never know), once you're into aftermarket gauges, they have their own wiring that does not use the IVR. When using OE Ford gauges, wiring, senders, the IVR plays a role. Usually, if you had factory gauges, the fuel quantity, coolant temperature, and oil pressure gauges (NOT the amp gauge) would be routed through the IVR. And if the IVR went bad, usually you would get all three gauges malfunctioning, and in the same way, such as all three needles going off scale high, or something like that. One word of caution on AutoMeter gauges (I have 3 cars/trucks with them), they have a Ford set using Ford resistance values which isn't as accurate as I would have expected from AutoMeter. Apparently they also have a non-Ford set which can be used that is more accurate, but the gauge markings are not quite symmetrical. At least that's supposed to be true for classic Mustang applications. Don't know if it applies to trucks. So you might want to call the AutoMeter tech guys and inquire. If you get any good information, let us know. Good luck!
'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