I got some info from cluster mods and its exactly what I suspected!!!!!!
He created a custom software that has the CAN data for speed and revs. BA/ BF and FGs all use CAN for this. The service manuals even have the CAN information you need to do this.
My only concern is that if I connect a dash to external power, a bench power supply for example, that the FG dash will freak out because it won't be able to read any data from the PCM, TCM etc.
Creating a device that goes from USB to CAN isn't all that difficult. But I don't want to lock out my dash, if that's even a thing. luckily I have a spare dash now to try this on, so hopefully I can make a test jig that mimics the speed, revs, fuel, coolant temp, etc so I can properly set the gauges.
Seems like the process he used was to set a specific rev and speed, and adjust the needle manually until it matches. Slow process, but it'll work.
So far, my dials aren't far off. If my digital speed shows 50km/h, the dial reads 49