I've worked on cars for over 50 years... but never had this happen!

Mid seventies Econoline truck chassis with 460.

I left the headlights on, and killed the battery. It was several years old, so I just replaced it. Now I get nothing at all. Guages don't come on... engine doesn't spin... lights won't turn on...

YES! The battery is fully charged, and hooked up properly.

Maybe a relay or something? What would a dead battery cause?