You're talking about using two transmission mounts as spacers to lower your trans crossmember, right? With the crossmember "hanging" on the mounts?
It would probably be just as good, and cheaper to use some kind of metal spacer to lower your crossmember. The rubber isolator that the transmission is bolted to should be sufficient to reduce vibrations or whatever.
If you like the idea of using rubber spacers, maybe some body mount bushings could be used as spacers. I don't think I'd rely on the trans mounts to support the weight of the trans and crossmember though.