If you're not looking to yank out ALL the wiring and go from scratch I'd put the original stuff back in. If you put a new one in over top of what you already have you could wind up chasing a ghost if you run into electrical issues down the road. Most good aftermarket harnesses have GM colored wiring, and even have the name of the circuit printed right on the insulation to make things easier. Be sure to keep your old wiring though because you may have to reuse some of your original connectors, or certain circuits altogether (for example if you are using factory cruise, the entire circuit comes out as one piece, wiring, vacuum hose and all... possibly the same for at least part of the wiper system as well if the wiper switch is on the dash). Most of the common stuff like headlights/taillights you can take the entire length of wiring out, and even get new connectors so everything will be new from fuse box to light socket. Personally I'm planning on starting from scratch on my '78... I bought it in pieces, so I figure since it's apart I'm putting brand new wiring, all stainless lines, etc. in so I don't have to go back in down the road and mess around with it.
