The only specialty plug I've used that actually seemed to help a tired engine was a "booster gap" plug with a break in the conductor back inside of the porcelain insulator. It's supposed to allow voltage to build up and fire an oil-fouled plug instead of bleeding off through the fouling. It doesn't sound like you've got that kind of problem.

Try doing a cylinder leakdown test by putting compressed air into the cylinder through the spark plug hole with the piston at the top of the compression stroke. The crankshaft is locked by putting the truck into high gear and setting the parking brake or using wheel chocks. Listen for piston ring leakage at the oil fill cap, or valve problems at the carburetor or tailpipe. Bubbles in the radiator would indicate a cracked head or a head gasket leak into the cooling system. All the cylinders should be within 10% or less of the same compression pressure, and 100 PSI or slightly less should be considered the minimum acceptable pressure. Be aware that different compression gauges can give widely differing pressure readings on the same cylinder, so be more concerned with uniformity between cylinders, not an arbitrary PSI reading.
Jerry

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