With a stock width A-body 8 3/4 and BBP axles installed it should be about 6" from the face of the drum where the wheel mounts to the springs.
So, 4.5" of backspace, with a 275 you should have about 1" of tire overhanging the lip of the rim at the section width, so that's 5.5", leaving a good 1/2" to the springs for clearance. That's how it should work.
Sounds to me like someone shortened your axle.
