Installing an aftermarket tach on an Inline 6

[longorange68]

Got a 1977 250 I-6 that came in my truck when I got it. The only reason I know this is because when I went to the Auto Parts store I brought the rotor and cap with me and they said so. I just swapped rear end gears to 3.73's and I was curious of my RPM's at highway speeds. Went to Schucks and bough an el cheapo Equus tach to mount on my steering column. The connections are as follows, one fused 12V power, one tap off of the gauge illumination circuit, one to chassis ground and the other to either the tach port an an HEI style distributor, the negative leg of the coil, or one to a standard ignition coil. Got it all hooked up except I am not exactly sure where to put the final wire that hooks to the ignition system itself. No tach port that I can see on the distributor, and I can't tell what wire is negative on the coil. I have looked at my Haynes manual, and been on the internet for an hour trying to find out what I need with no luck.