Distributors don't care about voltages or polarity; coils do. I would check with "Charlie ny" on the price and core requirements. If you plan on running a strictly stock engine forever, I properly set up "Load-a-matic" will be just fine. If you are considering any mods to the engine (particularly carburation), a converted SBC-type distributor is a must. If you are going to stay with 6 volts, stay away from Pertronix units as they are notoriously unreliable. I personally would not use a Pertronix even with 12 volts, but that's just me.
If you are going for a conversion, you may want to consider having an aftermarket "racing" distributor converted. I had Charlie do a Mallory dual point for me that works just fine. In my opinion, they look more appropriate in one of these engines than a late model GM unit. I guess the conversion is a little easier, too. You may be able to find an aftermarket distributor made for a flathead, but they tend to be expensive.