due to the age of your truck it is most likely that you have a vacuum leak
you need to check all the little rubber air hoses and rubber caps that connect to the carburetor and the intake manifold, replace any that are cracked or do not grip properly
vacuum leaks can also occur at the carburetor-to-intake gasket, intake manifold-to-cylinder head gasket, or inside the carburetor itself.
You can find them by listening for a whistling or hissing sound, with propane, or with carb cleaner.
When propane or carb cleaner is sprayed on the leak the engine RPM will rise
If your brake booster is blown it can cause a vacuum leak inside the unit and will be hard to locate. To test this remove the booster's vacuum line from the intake manifold and cap it off, if idle improves the leak is in the booster
The following will also cause a poor idle:
A stuck/leaking EGR valve
bad ignition system (plugs, cap, wires, incorrect timing)
a cylinder head valve that does not close completely
start with the vacuum hoses and work your way from there
