Here is a pic mocking up the 15x8 Wheels and 275/60/15's to see how wide I need to make the rear differential. To run this setup I need a 56 1/2" wide wheel mounting surface to WMS rear diff.
I'll be doing the same thing when my Dart comes back from paint. I'm going to center the 275/60/15 tires and wheels in the wheel wells and then measure wms to wms and then get the B-body 8.75 rearend narrowed accordingly - also will get the leafspring mounting pads relocated to fit the offset shackle kit. I'll be running these:
I like your build plans and will be watching them progress.