Make sure the points are properly set. If the points are either too close or too wide, you can have ignition mis-fires as you increase the engine speed, but nothing will show up on the spark plugs or points.
Also, check the ignition current if you have an ammeter. You should get about 4 amps of current with the key on, points closed, engine stopped. If you don't have an ammeter, measure the voltage at the coil positive terminal to ground, key on, points closed, engine stopped. You should find about 6 volts there. A little variation, a few tenths of a volt, above or below is not critical. Low voltage at that point indicates low current or high resistance in the primary ignition circuit.
Also, make sure you don't have a coil with an internal resistor connected to the ballast resistor. Measure the resistance between the coil primary terminals. You should find about 1.5 ohms, a little above or below is not critical, but if you measure something around 3 ohms, you likely have a resistor coil, which you probably shouldn't have. I believe your truck has a ballast resistor wire layed in the wire harness. If your truck has the harness resistor, one of the wires connected to the coil positive terminal will be stiffer than the other, and originally it had a white fiber glass shroud around black insulation on the wire. In most cases the fiber glass shroud is missing for an inch or so, so you may only find the black insulation and the stiff wire, which is the nichrome resistor wire.
You could have carburetor trouble, as well, but one thing at a time at this point.
Harvester
