Not trying to be funny here, but you DID check the fuel tank shutoff under the truck didn't you? Crap in the fuel tank, fuel line, etc. is very common in these old trucks.
edit: Just to add a little humor here, I used to work at the Chevrolet foundry in Saginaw, MI where the L6 and V8 engines, cylinder heads & transmission cases for Chevy trucks were made. When I started there in 1965, the AD trucks were 10-15 years old, and everyone was driving them for "work and back" vehicles - you could buy a good runner for $200 - $300.
A favorite trick was for some one to zip out into the parking lot and close the fuel shutoff valve on their buddy's truck. He'd get about a quarter mile & the truck would die in the middle of a big traffic jam. We all carried an old oil bottle full of gas to prime the carb with. Also handy for cold weather starts.
