It's unlikely that the auotmeter speedo will have the same parameters as the OEM speedo did - personally I wouldnt hook it up to the factory wiring harness unless I knew for certain it was compatiable, if you do, you might just fry the new speedo.
I'm sure autometer makes a speed sensor for that transmission, it might even come as a kit and be plug and play> 8)
You got that nitrous hooked up yet?
Chow> Cp