I just wired my tach feed to the stock tach feed and it worked right out of the gate. I spliced in to the feed that trips my shift light.
If you think you might need a tach adapter, I can get one out to you. I have 3 or 4 made up and sitting around, I thought I might need one and while I was wiring things up I figured it would be a waste to leave the spare resistors rolling around in my desk. I can toss it in an envelope and get it out to you at no charge.
Here is a screen shot of the Archangel manual of the wiring from the speedo/tach I bought. Ignore the connection diagram as I took the speedo's connector off and used a 9-pin male and female connector from radio shack.
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