What Gearhead said, and on a GM rear add 2.75" X 2 (could be 2.81") for axle offset and you get 60.5" overall axle flange to alxle flange. Also, keep in mind if your swapping to a Ford, the axle offset is only 2.36" to 2.50" per side, so there will be up to one inch difference in over all width if the housings measure the same width.
Just, just helped one of our members with this exact issue last week.
"But my original 10 bolt (2.81 offset) was 54" housing so I bought a Old Style Big Ford rear that is 54" and now my tires rub on the inside"
When your stuffing 10" wheels under there a 1/2" difference per side can be a mile!
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Frank Serafine
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