Stock B-body is 44" center to center on the perches. Yes, people have stretched the springs apart on their A-bodies and just used B body rear axles, but that forces the spring out of alignment with the spring eyes and shackles, putting a side load on both. Does is "work"? I suppose. It'll trash the spring eye and shackle bushings and cause binding in the spring action, which is already a draw back with leafs and shackles. And in that case you're actually reducing your tire clearance to the springs, so, you're not doing yourself any favors.
The 3/4" MP offset kit does a little of the same thing, the front hangers are offset a 1/2" (that's all the room there is to the frame) but the shackles are offset 3/4", and if you weld the perches at 41.5" c-c you're placing a side load on the spring eye bushings. In that case I suppose you could angle the perches slightly front to back to make the perch more parallel to the spring again to take some binding off the spring itself, but the front hanger is still drilled straight across so the load is still put on the bushing. The hanger is set up to be perpendicular to the spring, so if the spring sits at an angle it has to come from the bushing.
E-body springs are not perpendicular to the axle, they sit at an angle front to rear (the distance c-c on the hangers is smaller than the c-c distance on the shackles). That was done for some "turn in" advantage, gets into suspension geometry and how everything reacts when it's loaded. But it's important to note that they were designed to be that way, meaning, even though the springs sit at an angle all of the hardware is either parallel or perpendicular, the angle doesn't come from side loading the bushings, it comes from how everything is mounted to the frame. The rear frame rails on the E-bodies are actually splayed, they're not parallel to each other.
