If it's popping thru the carb, check the bushings in the distributor, a lobe is worn on the cam, or timing chain is worn and jumped a tooth. I would check cap, rotor, plug wires, and plugs first. Grab the distributor shaft while the rotor is off and check for side to side play. If that's good, disconnect the power from the distributor so the truck won't start. Pull the valve covers and watch the rockers while someone cranks the truck. If any aren't moving as much, your cam is junk. You can watch while it's running if you're alone, but it makes a mess.