The unit pictured in the link I provided earlier is the "analog" instrument cluster and has a mechanical speedometer/odometer. There is also a "digital" cluster that I have not seen, but the images that I've seen appear to not have any mechanical needles like yours.
I believe the key to finding the antislosh module is that it is a separate board from the instrument cluster and plugs into it.
FYI, in the analog cluster, the gauges (tach, temperature, fuel level) are magnetic devices that depend on the current ratio running in two coils. In general the two coils are wired in series with one contact to B+ a contact to ground from the second coil, and the sensor connects to the common node. The gauge deflects according to the amount of current drawn out of the common node. See this youtube video for a good explanation
https://www.youtube.com/watch?v=a2vb4oVdOCk
My guess is that your gauges read high because there is a relatively high resistance to ground. Although there are multiple ground wires connecting to the cluster, these join to a common wire before connecting to ground.
Maybe you can find an online picture of a cluster that is a closer match to yours.