Instrument Cluster Removal

The IVR-4 is correct for your '66 Dart.

Don't know what current (hah!) prices are for the voltmeter conversion; check with the two outfits linked above.

Your speedometer reads incorrectly almost certainly because you have the wrong speedo drive pinion for the car's rear axle ratio and tire size. If you'll lettuce know what size tires you have and what rear axle ratio, we can tell you what drive pinion you need. To figure out the car's rear axle ratio, see here. You very well may still need to have the speedo head rebuilt (it's over half a century old), and will surely need to lubricate the speedo head and cable in any event.