First off, did you check a lamp interchange chart??? What I mean is, are these the correct bulbs? My '70 uses 4001 and 4002, is 4000 correct for your application? If for some reason these are 6 volt bulbs, then we've solved your problems.
If according to the owner's manual, the bulb is correct, then it does sound like we have a voltage problem. You need to take a voltmeter and connect both probes to the headlight plug wires, turn on the lights and engine, and see what kind of voltage your getting. Anything higher than 14.2-14.5 volts is a problem!