I havea 1980 Ford Bronco sitting on a 1979 frame.  I had it running the other day and as I was fixing to leave it died.  I've changed the voltage reg., ignition switch, coil, ignition box on the fender and nothing works.  The only time I get spark is when I'm letting the key off and then it's only for a sec..  If anybody can help me PLEASE do!!  

One thing to check is make sure you have power to your starter solenoid on small wires on forward side when igniton key is on. If no power maybe a blown fuse or possible bad solenoid need to trouble shoot this out. If you crank engine it fires and dies immediatley when you let off This is where I would start. I had a solenoid go bad a couple of weeks ago and this was my situation. bad solenoid. Once had a bad wire to solenoid and took awhile to figure out.