42RH / A-500 in a Duster with pictures.

IMHO I think that you clearly have no idea how heavy this cross member SHOULD be
Bellied out the middle of the old crossmember, heated and welded 3/8 plate around the bottom and sides of the old xmember to add the strength needed. Did it in 1994 and it worked for years without issue.. I just used a chevy style trans mount sandwiched between the 518 top trans mount plate and the xmember itself.