"GB Sisson" mentions problems with voltage drops on the stock gauges. This is the one thing that has stopped me from converting to 12 volts. I am just uncomfortable with the concept, especially with the King-Seeley gauges used by Ford. With the advent of the Optima battery, I have never encountered a problem with a 6 volt system. I think that this may be due in part to the fact that my '51 was an extremely well-maintained 63,000 mile vehicle when I acquired it. The wiring was in excellent shape, and since I have replaced the original starter (that turned out to have a cold solder joint on one of the brushes), it spins over like a new car.
As far as 12 volt accessories are concerned, I made a 6 volt to 12 volt power booster and polarity change device. I now am able to run a backup camera, GPS, CD player, phone charger, and anything else I want. The only thing that won't work is an amp that draws lots of power. It turns out that the polarity change is unnecessary since all of the new stuff comes in plastic cases and needs two wires to function anyway. This, and the fact that the new stuff draws minimal current simplifies the process and allows the use of a cheap voltage booster available almost anywhere.
I am building a flathead powered "T" tub and that car will be 12 volts, as I am using aftermarket 12 volt and mechanical gauges. Because of this, I am comfortable running 12 volts. Then , all of the other justifications kick in.