Car is dead..help !

Went to start our '75 Duster, 360 with orange box ignition. Was driving it last night.

Thought we left something on so went to boost it...still dead. Won't even turn over.

Tried putting her into gear..thought that the only start in neutral may not be engauged..

The car doesn't have a ballast.

Absolutely nothing works....almost like having no battery in the car at all. The batter sparks (when I cross the booster cable ends) so I assume I have some current in her..

If the battery is truly farked, would it act like this ? wouldn't jumping by pass the dead battery.

fuses maybe ?

Any help is appreciated.

Ian.