1979 Mustang 5.0 won't start

I have a 1979 Mustang 5.0, automatic, current engine (not the original, but similar with a cam) has about 16,000 miles on it. I drove it about 100 miles without incident, parked the car in my driveway and the next day it would not start. When you turn the key, absolutely nothing happens (no click like a dead battery, no "whirr" like a bad starter, just dead silence).

The battery is fully charged, we replaced the starter relay and the battery cables. Lights, etc all work, and the car will start if you run a "jump wire" from the battery to the starter, so the starter doesn't appear to be the issue either. We also checked the ignition switch assembly box -- everything there appears to be properly connected.

I can't find anything in the Chiltons that's been too helpful (this book is pathetically organized). Things seem to be functional based upon the fact that we could get the car to start and continue to run, but the key in the ignition switch doesn't appear to be getting the message that it's time to START.

Any suggestions as to what else to check?