1964 Dodge Dart 270 Slant Six Restoration

I pulled off my instrument panel last weekend since my gauges are all non-functional or barely functional.

The speedometer cable is new and is connected to the speedometer. I spun up the speedometer by hand with the cable detached and confirmed it works, but when running the speedometer does not work.

The fuel gauge will say 3/4 full, but after about 2 miles drop down to 1/2 full, and then a few more miles it's at 1/4. It'll stay in the 1/4 to 1/8 range for about a week of normal driving, but I always put in $20 no ethanol gas every week, if it feels like it's starved for fuel, I do it a day or two early, but that's been only once it's felt that way. When detached, the gauge only had one of three connectors attached.

The thermometer has no wires hooked up, but the alternator is properly connected.

Does anyone have any advice about the speedometer or fuel gauge? Perhaps I need a new sending unit, not sure where is best to get one of those.

I didn't see a wiring diagram for the cluster panel in my FSM, but it's definitely possible I missed it.

As always, any advice is much appreciated, y'all have been very helpful!

Likely it has an aftermarket sending unit as your description fits. They are all crap.
Speedo, you saying if you spin the cable it works but in when connected to the transmission it does not? The speedo gear in the transmission may be installed incorrectly. In the FSM you will see the housing that hold the gear can be installed in 4 positions and depending on the number of teeth on the gear (set by rear axle ratio) the housing is rotated to a particular place. If wrong the gear can be pulled away from the drive gear and they don't mesh.