Sorry for the delay Jbalgiere. With a factory PCM your output should still be the same type of grounding signal. I was able to verify this by attaching a factory cruise unit to my Terminator VSS out and it works.
The Optoisolator board that I got is this one
here. Looking at my circuit diagram, it looks like I showed the original connection to A3 which is wrong. The center OUT pin on the optoisolator should connect to D3. VCC connects to 5v power, could be VCC on your ardiuno. GND is self explanitory. Then on the other isolated side of your isolator you'd connect one pin to 12v vehicle power and the other to your VSS signal wire. The polarity is marked on some of the boards so just know that 12v is + and your VSS is -. I would suggest using the Mode2 code for this since it just works a lot better.
As an aside, I've now been using this speedo for over a year and about 2k miles. Only issue I've had is that the home command for the stepper sometimes leaves the needle a degree off and the speedo may read 1-2mph low for that drive as a result. Fix would be to probably put a stopper for needle to rest against when being put to the home position. The miles are still being recorded accurately though.