Tell me whats wrong with my 1966 chevy

[NCD1966]

first off I dont drive my truck very often. Recently I had the truck at my property and wanted to drive it to my house (15 mins away)

the truck started up and ran great for about 20 seconds and just shut off. like I turned the key off.

So I ended up just putting it on the car hauler and brought it home.
did some checking and its getting voltage to the distributor (HEI) but no spark to the plugs. checked the power to the coil its at 12 volts on the "on" key position then I got to turn the motor over voltage goes to 9 volts. Also when the key on the tail lights come on and my tach light. Which that is suppose to go off the light switch.

I know its a switchI need to change just which one?