Unbolt the catalic converter at the front flange. If you break a stud drill it out and use a new bolt and nut. If it still back fires use an ohm meter and ohm all plug wires. They will cause a back fire. Take a compression test a bad lobe on a cam or bad spring will cause a back fire.
To check for a stretched timing chain pull the distributor cap rotate the engine by hand past tdc about 60 degrees. Then turn back to tdc use white out and mark the tip of the rotor and the dist. base. Have a friend watch the rotor as you turn the motor by hand stop as soon as he says it started to move. If it took over 20 or 30 degrees you need a new timing chain.
I think you have some type of timing problem. A bad vacuum advance or bad temp. sensor if computer controlled can also cause problems. With the age of this truck I think it is a mechanical problem.