When I start my truck it will run for awhile

Losing fuel or spark.

Why didn't you include those details? What else have you checked and what were the results? Have you actually determined if you're losing spark when it stalls? Can you restart it right away? Can you restart it after cranking quite a bit? Will it start after sitting for five or ten minutes? Is the stalling consistent as in it happens every time you put it in drive. Does it happen in reverse too? Which engine do you have? We need to know that when discussing an engine running problem.

I have that engine in two '80 Volares.

First unplug the vacuum hose from the distributor and plug it. If the engine starts and runs when you put it in gear, suspect the pickup coil inside the distributor. It sits on a plate that rotates with engine vacuum to achieve better fuel mileage and performance. That movement causes the wires to flex and eventually break.

Also, the next time it stalls, do not crank it. Remove the air cleaner lid, look down the middle of the carburetor, and see if there's a solid stream of fuel when you work the throttle by hand. If there is not, a common cause is a collapsing strainer in the fuel tank. Based on how you said it's acting though, I don't think you have a fuel problem. I suspect you're losing spark.

Also try wiggling the wiring harnesses while the engine is idling. The easiest way to determine if losing spark is the cause of the stalling is to connect an inductive pickup timing light to the coil wire and watch if it stops firing when a helper shifts into gear. You'll only have one or two seconds to notice that before the engine stops rotating.

Look at the connections on the ballast resistor on the firewall. That is used when you have the four or five-pin electronic ignition module on the firewall or fender. I think that was used too with the Lean Burn Computer bolted to the air cleaner housing. Those wires get tugged on when the engine rocks, as in when shifting into gear.