Your negative cable on the battery is either bad or not making a good connection, it should be connected to a bolt on the block on passenger side in front of the motor mount (don't just do a visual inspection, wires can go bad inside the crimp), this is your problem, or you have a bad starter drawing too much current (less likely). And anytime you start your car, remember, the starter must cool, so if you crank the engine say for 30 seconds and it doesn't start let the starter cool for 5 min before you try again. And you should also have a ground wire on the passenger cylinder head to the firewall, this is a body ground (it may be different on a 66). Good Luck.