I'm considering an A 500 4 speed overdrive for a 72 Duster

Boy THAT IS an interesting approach to the crossmember. Other's I've seen cut it clear out and install a bent piece of thick strap. Seems like your approach might be somewhat easier, and maybe combine the two methods, IE lay a piece of bent strap up into the cut channel.
Yep, my thought was to cut/channel the existing cross member, hoping it retained the most amount of rigidity. That coupled with the uscartool 3/16 trans member, I'm hoping it all holds up to the stroker.