If somebody else needs the data point I've measured mine now. 1981 F-250 Custom (mfr'd 3/81) 6.6L V8 with the 8650 GVWR. I have a Dana 61 with a semi-float 30-spline axle and 9.75" ring gear, 3.73 ratio, axle capacity is rated 6250.
With all that said (I've learned to be VERY specific!) I have 3" shoes. It's really hard to tell from the photo or part number but just from a rough guess I THINK those might be 2.5". They look narrower than mine but it's hard to tell from the photo. If you want them you may want to contact the seller to confirm.
I purchased
https://www.amazon.com/gp/product/B000C9FQ44. I haven't installed them yet, just got them last night and it was too dark for driveway work (I'll report back here today if there's an issue). But I held them up next to the old and they look like the right parts.
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1981 F-250 Custom. 6.6L V8, 4 barrel Holley carb, ARA aftermarket A/C.