Starter drain the battery dead???

I assume you pulled all the fuses, but you likely know there are other items in these old girls that are not fused. A good study of the wiring diagram might give you a hint. Don't forget added items, the radio (modern radios run the output transistor(s) "hot" all the time, and they are supposed to be "cut off" when the radio is shut off. Double for remote amplifiers. Glove box lamp, trunk and underhood lamp. Someone mentioned the alternator, just disconnect the output stud. Pull the horn relay. Don't discount that the battery itself might be iffy. Charge it up and store the veh. diligently with the neg cable disconnected. Obviously if it still goes down, its the battery.

Sosmeone already mentioned, but make absolutely sure that the brake light switch is not on the ragged edge, and sometimes sticking 'on'

Did someone connect something like a tach and wire it "hot?"