On my '47 Plymouth converted to 12 volt, I am using the original gauge and sending unit. If your original sending unit is working fine there is not need to replace it.
I've got a runtz resistor wired in series before the hot wire on the back of the gauge. That is the one coming from the starter button, which is getting it's power from that feed wire. 12 volt will most likely damage your 6v gauge, but with the resistor it works great. Sending units are not voltage sensitive, but the gauges are.
If your sending unit is bad then there are several folks here on the forum that have used a 12 volt alternative, seems there has been a lot of talk about that in the past, so you may do a forum search to read up on that information.
If you are looking for a new fuel tank rather than a used one, the Tanks, inc tank is a good choice and after looking at their website they do offer a new sending unit you can purchase as well. Not sure if it is compatable with the original fuel gauge though.
http://www.tanksinc.com/index.cfm/page/ptype=product/product_id=226/prd226.htm