Choices for reproduction batteries have become very limited in the past few years, but what you are asking for never existed. Those were 6 volt cars. When you put 3 cells in a battery for 6 volts, and connect them with lead bars, the posts wind up on opposite corners. When you put in 6 cells for 12 volts, the posts wind up back on the same side. The three caps on the 6 volt battery are for the three cells, so a 12 volt battery necessarily has 6 caps on it because there are 6 cells.
Some of the last repro batteries were fakes with a sealed or semi-sealed battery inside, like an odyssey or something similar. I guess it would have been theoretically possible to make a battery that looks like 6 volts on the outside with a 12 volt sealed battery inside, but I have never seen something like that advertised and I doubt it was ever offered.
If you just want it to look old, you might look for a repro battery for an early 12 volt Chevrolet, like a 1955 or 1956. It will have 12 caps and the posts on the same side. You will need about one more inch of length in the tray than you needed for the original battery. If there was some space at the ends it might even just fit.
If you want it to look right, run 6 volts. Even then I'm not sure you can still get a reproduction battery. Maybe. You still could about 3 years ago.
