You have some really good points here to check on. Pull out a spark plug or 2 & see if there is black soot on the porceline or the edges of the shell. If it is you are getting to much fuel which I suspect being that poor of mileage. Pull the top off the carb & check the float level & make shure no gas has leaked into the float. Check the power piston goes up & down. See what size jet is in it. usually 053-055. Sometimes a PO may have changed it or drilled it out. The vacuum advance should move when you speed up the engine. You are either not getting vacuum to it or the diaphram is bad. The weights they are refering to are separate from the vacuum & are called mechanical advance. They are down inside the dist. The dist rotor should turn a little if they are not stuck. The higher gears are great for these old trucks but you need to find out the problem first. You can verify the speedo by comparing the odometer to the mile markers on the interstate.
