Guess he already has a low amperage converter for the gauges?
Will need to replace the coil and condenser in the ignition system.
All the lights. (is there a headlight relay and a horn relay?)
Hydraulic/electric brake light switch if they use one like the Tbird probably would be ok.
Fuel sending unit, starter, wipers electric or vac like the Tbird?
Bet someone makes one but I didn't see it in a quick search - think it would be possible to find a high current 12v to 6v regulator. Wonder how much input voltage his old mechanical regulator can handle? Just send the 12v to that and use the 6v output on the windows?? - 12v might overload the old 6 v reg though. I know the old Lucas 12v adjustable mechanical regulators on the MG's will go down to about 8 volts output with 12 volts in. Believe they are 30A so that should work for the window motors. Have to make sure it's a fully adjustable unit - they are making solid state replacement units now.
re:Lucas - Maybe I'm related to Lucas aka "the prince of darkness" but in 30 something years of owning British cars the only electrical failures I had were on my '79 MGB. The plastic headlight switch fell apart in my hand one night on the way home from work (just took the connectors off the switch and crammed them together) and a bad ignition relay that I did have to replace.
Just some ideas.....