How concerned are you that it 'looks correct' ?
A readily available 12v battery that will fit may be hard to find at the corner auto parts store, especially if you want it to look 'normal'. Depending on which model vehicle you have a '55 6v battery tray can be long but narrower, reducing your choices. A tractor or industrial supply place could be an option.
Have you considered changing the battery tray to one from a '56 or '57 12v car?
They can be a little wider, and you'll need new battery hold-down hardware to fit the battery.
My '55 has a 12v Interstate SP-22NF battery in it, the widest one that would fit.
The battery is 9 inches long, 5.5 inches wide and approx 9.75 inches tall.
It looks like I borrowed it from a riding lawn mower.
With a heat shield installed the smaller size may not be too noticeable.
This battery is pushing 6yrs old. Your question reminded me it's almost time for a new one.
The tray under it may not be original to the car and doesn't fit quite right either, so a '56/'57 battery tray may be a good option here too.
And remember to double check the actual measurements of the new tray and battery.