A negative ground electrical system will have the hot wire from the ignition hooked up to the positive (+) side of the coil, with the negative side (-) to the distributor. Your truck (57 Chevy)should be a negative ground. If the wires on the coil are reversed, you'll dramatically shorten the life of your points.
Just to be sure, check your battery cable that goes to the starter....it should be the + terminal. The negative terminal should go to ground. Hope this helps.
