As Andrew said, 15.5 is probably too high, your idle is too low. Your coil voltages look ok and where you have checked I do not see a voltage drop issue.
I would say voltage drop to the voltage regulator on the sense side needs looked at, usually it's a blue wire but it is the switched 12V feed to the VR. Measure that to engine block and it should be the same as the battery + terminal or alternator output stud voltages, if not you have an issue if so it's probably the regulator.
In 68 you should still have the single field terminal alternator with the mechanical regulator system, unless it has been changed.
If you are still running the mechanical voltage regulator instead of the later electronic version of it (not to be confused with the 70 and up regulator) you can adjust turn on and turn off voltages. To tell if it's the mechanical regulator look at the bottom, if it has a couple wire wound resistors it's the mechanical one. The FSM has adjusting procedures in it.
No truck FSM's, but the 68/69 pass car ones have the relevant info in themhttp://www.mymopar.com/index.php?pid=109
I would think about converting to the newer charging systemhttp://www.mymopar.com/index.php?pid=78