few things i need help on with the 318 in the car

Yea. probably a fan belt, ol man's probably nailed that one.

Car should idle at...what is it. Stock? Modified? Automatic? Stick? Mine idles in the neighborhood of 900rpm fwiw.

And, rise as fast as normal? Is it slower revving because you added a tach. Or seems slower because you can see the rpm now? Could be bad gas, air filter, fuel filter, timing, carb tune, vacuum leak, temp is too hot, choke is closed.....

I doubt you have a wrong wire, there are 4cyl, 6cyl and 8cyl, settings on a tach. but you'd have to usually cut a wire or turn a knob to change from 8cyl.