I'd say about 56 57" hub to hub ideally...... I kinda departed tradition and am running a wider 60 Galaxy rear[61" hub to hub] in my 30A roadster.....One benefit I discovered-whislt drivin it in the rain , the tires dont throw **** on the windshield, OR the back of your head!
Stock Model A track is 56 inches. Most late 50's cars run around 59 inches (57-59 Ford, 55-57 Pontiac, Chevy, Olds) and work okay. Choices seem like they're pretty wide open, later Nova/Chevelle stuff works too if you don't mind making mount brackets. Stay away from S10s, they're too narrow (54.5 inch track). There's a number of threads on this, I posted one last night -
57 Ford rear axles are 57 1/2" outer brake drum to outer brake drum. (Which is the commonly accepted way to measure them.) 56" as Choprod suggests is the usually recommended width for most hot rods. Especially so the thin fendered models - A, 32 etc. 56" axle width works well with the common wheel backspacing of 3" + or - an eighth or so. That allows wheels up to 8" in width - with the correct backspace, which, again is about 3" - to fit under the fenders if a reasonably sized tire is chosen. Even so, a 6 - 7" wheel width can be a touch better to use if you're using a tire a little less than large by huge. As a fwiw, my 32 highboy has a 55" wide rear axle. The car runs 4 bars between frame and tire. There's never been a problem with rubbing which is a touch surprising considering the rear tire on it is a 285/70R-15 on a 10" wide wheel with 2 7/8" backspacing. The pic below shows the rear view of a 31 Model A roadster on 32 frame. Tires are 235/80R-15. About 28" tall and 6" tread width on a 6" wheel with 3" backspacing. Note as well that the right fender is 1" wider than the left fender. The left fender is a bobbed 30-31 fender and the right fender is a 28-29 fender. Short story on these, I bought them and didn't realize they shouldn't nest together as well as they did. They were already bobbed and off a 40's era roadster and came out of a 100 year old barn so they have some history. Not 100% sure which way I'm going to go far as 28 or 31 style fenders go, but I'll probably opt for the wider 29 for the added tire clearance on the side. You can tell the difference between 29 and 31 fender styles by the width of the indent for the frame cover above the running boards at the front of the fender. Once the fenders are bobbed - except for the slightly different bolt pattern - it's difficult to tell exactly what you do have. I'm fortunate to have another set to bob so I can at least have a matched pair. Matched pairs be good....
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