Speedometer is not functional?

I've never done this repair but I read about it very often. Failed gauge stepper motors are extremely common on GM vehicles. Often they're sold in sets of six.

Before you pursue that, look for a small stop peg just under the "0" on that gauge, then check if the pointer is on the wrong side of that peg. Sometimes a voltage spike occurs, particularly when the battery is reconnected, and that spike sends the pointer over halfway around. Stepper motors are not like regular motors with brushes and spinning armatures. Instead, they have four electromagnetic coils of wire that are pulsed with varying voltages and polarities to position the pointer in the desired orientation. Nothing is spring-loaded to return the pointer to "0".

Once the pointer goes past halfway, when you turn the ignition switch on, the motor gets pulsed to return the pointer to "0', but it looks for the shortest way to get there, which now is clockwise. That puts it on the wrong side of that peg.

If that's what you find, there's three ways to reset the pointer to the correct side of the stop peg. The most tedious and time consuming is to disassemble the gauge cluster to the point you can push the pointer with your finger. We'll call that "the last resort".

A better method is to use a scanner to perform a gauge test function. That runs all of the gauges to 1/4, 1/2, 3/4, max, then back to "0". By 3/4, the shortest way for the speedometer pointer to get there will be counterclockwise. It will jump there, then follow the test back down with the rest of the gauges.

There's a still easier way to solve this. That is to drive up to whatever speed is straight across from "0" on your speedometer. Commonly that is around 60 to 70 mph. Once you reach the speed where the shortest way for the pointer to get there is counterclockwise, it will go there, then continue with normal operation.

Let me know what you find.

The gauge test with a scanner is only of value when you can see the pointer is on the wrong side of the peg near "0". If the pointer is on the correct side, observe if the odometer is counting miles. If it is, the speed sensor is working. If the pointer is on the wrong side, remember, you can reset it by just driving faster than the speed shown straight across from "0". You don't need the scanner.

We don't get involved with custom builds like you're asking about with some other kind of speedometer head. The better solution is to pursue the steeper motor repair kit. I'd expect to find a number of videos on YouTube to show the repair being done. I did find this video showing how to remove the instrument cluster on a GM truck:

https://youtu.be/_HEC44xENxw

Yours will be different, but it gives you an idea of what the job looks like.

Instrument clusters today are computer modules, and in many cases, especially on GM vehicles, they are the most intelligent, (complicated), computer on the car. As such, they will be built in such a way that allows them to be disassembled for repair.