You have to be careful about lean burn distributors. I think 1977 is lean burn era. Here are variations, but I don't know years, or application: locked rotor no advance, two pickups, limited advance, with and without vacuum pod. In testing I have done the VR signals, amplitude variations exist, resulting in timing timing among cylinders. The worst from dual pickup units were the signals mix due to flux leakage. The other variation on rotation angle due to runout, and remnant magnetism, from pickup magnet setting spot on reluctor.
If you DIY, using a Sharp optical vane switch, and Mercruiser reluctor works well. Points distributors good for core. I also like vintage Mallory Uni-Lite.