56"-58" is the ballpark. If searching for one out of a vehicle, one from a first-generation (1966-1977) Ford Bronco is 58", has a 5-on-5-1/2" bolt pattern, and decent drum breaks. But yes, depends on wheels and fenders.
The previous replies will determine the overall width you want, in addition to bolt pattern, and whether a centered pinion is desired. The early Bronco rear has an off set pinion. Centered pinion 9" include 57-59 Ford (56.5" wide, 5 x 4,5 bolt) 57-59 big Mercury ( 58" 5 x 5). Determine the width and pattern, and a quick internet search will give you some good choices. If not too much HP, an 8" out of a V8 Maverick is a good choice at 56.5"
Get the wheels and tires you plan to use and set them next to the body. Decide if they are wider than your outside fender lip (if you want fenders) or narrower than your outside fender lip (if you want fenders). If the outside edge of the tire is wider than your fender, you may need to cut the inner fender wells to move them in. If they are narrower than your fenders then move them out till they look right. The relationship of the outer tire edge to the fender means a lot about how a finished car will look. Even if you don't plan to run fenders, its best to plan for possible later installation of fenders on your open wheel car. Once you have found just how you want your tires positioned and you have the correct size tire and rim sitting there, then measure and see what width rearend YOUR car will need. You may (probably not) be able to find something exactly the right width or close enough to work......BUT you need to know based on the tires and wheels you plan to run. Lots of people have found that the tires and wheels they planned to run did not fit ....or fit well.....when they used different wheels/tires for their moch up.
I know someone who narrowed his axle twice; because he relied on specs instead of waiting for his wheels to show.
I installed a Mopar 8-3/4 rear in a '41 Chev for a perfect fit. Chev was just finish painted, so I centered it all up and mounted it onto the semi-elliptic rear springs. Left side was perfect, right was 3/4" 'in'. I measured again and again, plumb bobs, etc. Chevy friend dropped by, there were two different rear fenders on this coupe! ('46-'48 was wider???) Owner said to "fix it, don't touch my fresh painted fenders!" Buddy Jim at Quality wheel reversed some Buick outers, one 'dumped' deeper...Perfect fit. I'll never admit it... (laughter) Some people's customers!
I to like the early Bronco, it is the right width and it keeps the original big bolt pattern, especially when your building a traditional car using 40 Ford brakes up front. HRP
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