Thanks cudaman. I realize it'd be stronger welded to the floor pan everywhere, but there is considerable disagreement (over on FABO, for example) as to just how much stronger...
http://www.forabodiesonly.com/mopar/threads/sub-frame-connectors-please-build-them-correctly.286631/

What I really want to know is, could I cut a 2x3" hole in both the front and rear sides of the t-bar crossmember and slide the box tubing all the way through and weld both sides (as Monte recommended with the round tube)? Again, that would definitely be stronger than a plate connection to only the rear face of the crossmember, but will it be enough stronger to complicate the installation that way.