I am wiring up my four bolt main 383 in my 87 Iroc right now. I am wondering where the two power cables go coming off the starter solenoid hook up? One obviously goes to the battery but where does the other big power cable go to?

 

Here you go. good luck. The black neg from the battery usually runs to the alt bracket. Just as long as it goes to a GOOD solid ground you can put it anywhere on the engine.
Hope this helps

 

My '92 Z28 has three separate wires that go to the bigger bolt, and one that goes to the smaller bolt (I think, but could be 3 on the smaller, and 1 on the bigger). Any idea what the additional 2 wires are for?

 

The car.

That's where THE WHOLE REST OF THE CAR (almost) hooks to the battery.