428 electrical

[1970 f250]

I have a 1970 f250 4x4 it has a 428 cj in it. The rebuild has about 500 miles on it. It has sat for about 4 years after untarping the truck and cleaning fuel lines and putting the carb back on it. I started it. However it will only stay running with the key in the crank position. When I let off the key I lose electricity to the coil my meter says zero. If I turn the key off then back to run it shows power at the coil but as soon as I crank it and let off its almost as thought it trips a breaker. I can hold the key in crank and pull the starter off the solenoid and it stays running until I release the key to run then it dies. Is there a relay type breaker or is my solenoid bad or what the hell. Does anyone have any answers. answers.