1967 c10 with 250 inline won't stay running

[sush1024]

I left work today and the truck ran fine and went to the gas station maybe a mile down the road to clean the windows and then the truck wouldn't run. It would fire then quit immediately. I help the key to start and it would still run then as soon as i let off it would quit so I pulled the fuel filter made sure there was no gunk in it then pulled the sending unit out of the tank and checked for blockage and there was nothing. Then it fire up and ran fine now it's doing it again and I can't get it going.