Ok so lets really talk K-members here Folks

I can't speak for bill at RMS but I do believe he uses the mustang II for geometry reasons plus the fact that there are so many aftermarket brake set UPS on the market for them.
And Joe I believe you are correct with the common brake kits being a big factor and I'd bet he didn't want to design his own and pay for casting them up so he went with something he knew worked. I don't blame him at all for using the mustang 2 spindle it's very available and an affordable spindle.