Rear diff question

The narrowest of the 8" rearends that I have researched is 56". An A Body is 57 1/8" wide with 43" spring perches. I have seen that 56" 8" Ford swap, it was pretty much a drop in, except for the driveline. 66-67 Mustangs had a 57 1/4" wide diff in them with 43" perches as did early Mustang IIs, if ya like 4 bolt wheels like the Mustang IIs. I flogged the 8" in my 66 Ranchero unmercifully and never hurt it. Where in the world did a 53" wide 8" come from? I'm going the 9" direction myself. It's about the same width as the short B Body rearends. Spring perches are 44" centers, but the register hole in them is about an 1" in diameter, I think the spring contact area can be modified to put the centerbolt in the correct place with very little effort. The shock plates are the big question. The OE holes are on 3" centers and just guessing the 3" tubes on the 9" will require about a 3 1/2" centers. I have the technology to weld the holes in and redrill them. Anything I'm missing? 8 3/4s are great rearends, but way too many of them found their way to the smelters in a foriegn land better left unspoken.