Ok, image below is from the 1970-71 parts book.

No mention of spacers with the B,J models (Barracuda / Challenger) although the FSM clearly shows them.

Note the highlighted section for the B-body crossmembers says "w/spacers". (W/' is 'with')

So to me, this seems to indicate welded spacers for the B-bodies, likely because the floors did not have the cup in place that the e-bodies had to help hold them in place.