A '77 Chevy should have had an HEI distributor... does it? They're very easy to wire, one hot wire should do the job. I don't see why you couldn't use power from the original CJ ignition wiring.
All that is necessary is to supply 12 volts to the power plug located under the rectangle sticking out the side of the HEI Distributor. There's also a plug for a tach, next to that.