Suddenly no electric

I pulled the Slant out of my '74 Dart for a rebuild a few months ago, and just today got everything ready to fire it up again. The electrical system is all stock. Last night the ignition system worked; we got spark in a spark plug grounded to the cylinder head to set the "static timing" on the distributor. This afternoon we finally went to fire up the engine. The first two times it cranked but didn't start. Each time, I squirted more fuel into the carburetor. On the third attempt at cranking, "clunk" followed by no cranking and all the lights going out--no parking brake light, no oil pressure light, no dome lights, nothing. I assume the clunk was the starter relay. Troubleshooting we've done so far:

- The battery is still at 12.5 volts. It was 12.6 before we began.
- There are no blown fuses in the fuse block by the parking brake handle.
- There's continuity between the battery + and the firewall via the fusible link.
- There's continuity between battery + and the ignition coil. I think that means the ammeter is OK.

Any ideas what to check next?