Hi Sly, I see you're in Germany. I don't know what the driving conditions are over there, or what the fuel mix (gasoline/alcohol) is, but I suggest that you try Tom's fix of insulating the fuel line before you go to a fuel pump. Many have successfully utilized this fix over here in the States.
If you go to a fuel pump, I'm pretty sure that you are going to have some trouble finding one in a six-volt configuration, but I don't suppose it is impossible. You will certainly want one which is low flow and low pressure. Your biggest problem will be fitting the return line. Unless you remove the tank and install a new bung into which the return line can be attached, you would have to use a closed loop system which returned the fuel to the fuel shutoff valve location.
Now, just for your information, I have done this on my own 29 roadster. Why? I am experimenting with a throttle-body fuel injector - just for fun. The return bung is under the fuel tank on the driver's side of the car. Let me assure you that this will be a lot of work, and you are best off by insulating the fuel line like Tom previously suggested. Happy Motoring!