Tying in the 2x3" .125" wall frame connectors right next to the torsion bar sockets goes a long way to strengthen the crossmember. Welding in that tubular transmission mount should do a pretty good job of tying everything together, although I still think I'd be inclined to connect the ends of the crossmember to each other and the floor over the transmission with something, even if it was just flat plate in an "A" type configuration (think I saw that on an a518 swap thread). It's hard to tell from those pictures, but it does look like he boxed the ends of the crossmember.
Big problem I see with that Duster thread is- How in the hell is he supposed to remove the torsion bars? His transmission mount is welded to the frame connectors behind the torsion bar sockets. The bars won't slide out the way he's got it. Also, if the transmission mount is welded in solid (as it looks to be) it will make it a PITA to pull the engine/transmission later.
I think the best way to do this is to modify the bolt in section of the crossmember so that it still forms the lower transmission mount, and then box the ends of the crossmember and tie them together over the top and to the floor with some kind of plate.
This thread here is the way to go. It's an a500 swap, but its the same idea.
http://www.forabodiesonly.com/mopar/showthread.php?t=178459
You could also use a 200r4, there's a thread on that as well, and I believe someone is making the adaptor. Very little cutting on that one, but you need to spend a little money on the transmission so it will hold up.