I'm really beginning to think that's not a B-body rear, it's too wide.
Those are 11" drums so you have the 5x4.5" pattern. Do you know if they're 11x2.5" shoes or 11x2"?
Your measurement from the inside of the drums is misleading. BBP backing plates are offset by about 1.75" to the inside from the end of the housing. If you use 53-5/8" and add 3.5 you get 57.125" which would be housing end to end, which is no where near a B-body rear axle. That's a tough measurement though because of all the dents and features on the back of the backing plates.
Your other measurement of 3" from the spring to the inner sidewall of the tire is telling. If you take that 3", add the 4.5" backspacing, and the tire overhang on the rim which should be .6" by the manufacturers tire specs you get 8.1", which is wheel mount to spring. If you take a 65-69 C-body rear axle drum to drum it's 61.75". Subtract the perch distance of 43", divide by 2, and subtract half the width of the perch and you get 8.125" for the wheel mount to spring distance.
I'm thinking that may be a C-body rear axle.
