According to the plans, it is necessary to choose a differential prior to building the basic ladder frame, as the type of differential could influence the placement of the rearmost cross member.
After a spirited discussion of my options on this thread
Which differential for a scratchbuild? I've decided to go with a 8.8 inch IRS from a 2006-2010 Ford Explorer.
I'm wondering how difficult it would be just to find some Ford Explorer uprights/knuckles/spindles and adapt them to my car, so I can use readily available parts.