Tremelune: Are you thinking of a specific vehicle, or is your post about general applications? I agree in principal with your comments, and have thought along the same lines. Saw a guy last year throwing Subaru rear units under all sort of odd vehicles, but they were pretty narrow. I bought a Ford 9" rear end several years ago for a project in order to get better brakes, and a beefier rear axle, but it turned out to not be a positraction unit as advertised when it arrived. I sent it back. Since then, I've looked into various suspension upgrades, and have come to the conclusion that it is probably easier to just swap something in like you are referring to. Add in the EV conversion issue, and it might make sense.E30 has another spacious trailing arm setup...Track width is 56" (though I think I really want the hub face to hub face measurement):
Last summer I measured a 5 series BMW rear that was within 3/8" of inside tire to inside tire dimension of that same car, which happens to be a Unibody. I'm not confident about hacking up something like that, and I don't weld. Looking at various specifications, I've noticed that published track widths don't tell the whole story, and are only a guideline. It appears different specs are used, such as center of wheel, back of disc mounting surface, etc. A tape measure seems to be the right tool for the job.
I'd like to see some projects where a Leaf unit, or small Tesla motor is used with a rear end swap like this. Very interesting.