Okay well I figured out the one in the Chevy K20. I must've had the labels wrong and hooked the coil wire to power, and the power wire to the coil. I didn't even pay attention to the color. So I switched that around and everything works fine now.

On the 66 Bel Air, I redid the connections to the tach. Now I have power to it. The shift light works and everything, but the needle is still stuck at 500 rpm. It doesn't bounce or anything.