275/50x15 is hard to find everywhere. While many sizes were available 20 years ago, these have been whittled down to only the most popular these days. A 275/50 would be a very short tire, so not many people would want one. 275/60 and 295/50 are the two most popular large width 15" tires these days. However, you may be able to use a MT SR that is 26x10x15. This is about the equivilent of a 255/55x15.
Just taking a stab at it, a 275/60 on a 10" rim should have the tire sticking out about 3/8 to 1/2 inch per side past the lip of the wheel. With 1.25" to the spring pack, you have plenty of room. Even with it being closer to the wheel opening, you should still have room.
BTW, your 5.5 backspace may not be compatible with a B body rear because the housings narrower overall width means you need less backspace on the wheel. It used to be that finding wheels with high backspace amounts was not easy, thus the B body housing swap. Now days, high offset wheels are the norm so finding a large backspace to be compatible with the stock E housing is easier to do. However, you would still need to move the springs in some to avoid interference to achieve ultimate tire. Careful measuring before you swap should yield an answer.
BTW, when you say 1 1/4" from the spring, is that up front by the hanger or out back by the shackle? E body springs are angled and this measurement is not consistent front to back.
