Pony 66

I just picked up a 1966 Mustang Convertible, something seems to be draining the battery. Had it tested and charged (parts guys said the battery tested good) and reconnected the cables. Wouldn't start without a jump. Let the engine run for a few minutes, turned the lights on, the engine just shut off. I know if could be as simple as a bad battery, I'm not sure why the engine would shut off when turning the lights on

Gun Jam

First thing first. With engine running what is voltage at battery terminals. Should be 13.5 to 14V
check that before beating yourself up