The six volt system will work fine, but is less tolerant of lack of maintenance and poor connections. It is also harder to find six volt accessories such as stereos.
One would have to define performance in order to claim that a 3.55 rear end will increase performance over a 4.11. Everything else being equal, the 4.11 would be both quicker and faster in a quarter mile and in stock condition COULD even have a higher top speed because the 3.55 gears don't allow it to reach an effective point on the power curve. The 3.55s will make the driver more comfortable due to less engine noise and the engine may be happier at lower RPMs on the highway, but is that performance?
