Thanks for the diagram, Barry! It's hard to tell if the spacers are above or below the floor pan (unless I'm missing something, which is very possible... )

I'm beginning to get a sense that it's only the B-bodies that got braces with the welded spacer on the underfloor brace. It also seems like the 6-way tracks were more commonly found on the E-bodies, making it difficult to find B-body examples.