Poor spark can also give loss of power under load, even though the idle is still smooth and consistent. The reason is that the additional compression of fuel and air when the throttle is opened above idle "quenches" the weak spark. To check this, first measure the voltage of the + coil terminal (NOT the terminal that goes to the distributor) to chassis ground -- expect to get ~17V here during idle. If you have less, then the most likely culprit is the 4500 RPM box. This happened to me and I only had about 8V at the coil. I had smooth idle but no power, and even significant missing under load. If you get low voltage readings then unplug the RPM box plug (it has six terminals, two rows of three) and jumper the harness plug center terminal of one row with the center terminal of the other row. This disables all of the RPM box features and just gives full voltage to the coil and ballast resistor (the ballast resistor is why we get around 17V at the coil, and this is correct and expected).I am now back to the original problem - the loss of power under load remains?
You may not have any mechanical issues at all if the above is true for you. Let us know what you find. This should not dissuade you from doing valve adjustments and other "normal maintenance" work, they still need to be done if you expect a good running truck.