1986 ford f 150 has nower power anywhere and wont stay running?

okay so this is what i got 86 f150 i bought from my uncle he had changed the motor which was a 300/6 when he did he left all of the efi stuff off and just put a carb on it! so it ran fine for a few years now i have it i went out to start it one day and there was no power anywhere in the truck no headlights brakelights key does nothing in any position no radio i mean its dead so i tried to jump it with a screw driver it starts then dies as soon as i take the screwdriver away battrie is good i clean every ground i could find and all the positives as well so what do i need to do to figure this out thanks for the help!!!