I hit send before I was done. I have never checked the magneto voltage between pre Jan 78 models and the newer ones, but my feeling is that the magnetos produce the same voltage.
In the 6v system, a Zener diode regulates this voltage to 6v, shunting the excess back to ground. The 12v system just runs at 12 volts without the zener regulation.
I have a 12v July '78 Moby 50V with a pre-78 lower end (and magneto) and I had to modify nothing since the wiring on the bike never had the diode to begin with. The 12v bulbs run just fine. The magneto wires were a different color, but you can mix and match or use a VOM to find the right ones to connect.
You should have no trouble going the other route since your 77 bike already has the diode installed. The 77 is set up for 6v, and the diode will simply regulate the voltage at that level.
See what voltage your machine runs on (check the bulbs) and make certain the diode is installed, if needed. If you need to install the diode, go to any electronics store or www.digikey.com and order a 6v zener diode with a 50 volt rating that will carry at least 2 amps. The diodes cathode (end with the stripe) connects to the black wire behind the headlight, and the opposite end (anode) connects to ground.