I don't know if the modern flash units have external 6v connections, but in the old days (vivitar 285) we made external D-cell battery packs with a Radio Shack battery holder and a pair of fuses that were the same size as a AA battery. Sorry, can't remember the specs on those.
We made a small notch in the battery door through which we fed the wires from the battery pack. The wires were attached to each fuse, each of which were placed in the end slots, if I remember correctly.
The old Quantum batteries that provided 6v also required a notch in the battery doors. Many of the old flashes, though, had no external power connectors, so that was our only option. If your flash uses an RCA plug, which I find odd, then by all means use it. You may find that, with AA's installed, you can get a reading on the external jack to figure out the polarity. If not, the flash unit will have protective circuitry, so you can also do polarity trial and error (ou have a 50/50 chance) without fear of frying the circuit board.
As for voltage, 4 rechargeable D cells have 1.25V each, for a total of 5. Standard D cells have 6. Either will work.
