Crossmember from any 78-88 G-Body car that had the TH350 or TH200C will work. Malibu, Regal, Cutlass, Lemans/GrandPrix.
Summit Racing (part# SUM-770300), G-Force Performance etc.. have them also. http://www.crossmembers.com/g-body.htm
The 2004R crossmember will work for a TH400.