Two very important parts of narrowing a rearend are (1) getting the ends rewelded correctly so the axle flanges are perpendicular to the axis of the axles, and (2) making sure the rearend doesn't get warped by too much heat from the welding.
The previous owner of my '69 Dart project installed a B-body 8.75 that could still be used with custom-backspace wheels, but I already have the wheels I'm going to use on my Dart. I'm going to rely on a pro to narrow the rearend.
Good luck if you decide to do the narrowing yourself.
