John, I don't think that a rear crossmember is required judging by the various transmissions that Ford hung off the back of the truck bellhousings.There were T-98 4 speeds and F-O-M's and Cruise-o's.
If you do decide to add a crossmember you should be able to leave the existing mounts and just support the tailshaft.
http://ford-y-block.com 20 miles east of San Diego, 20 miles north of Mexico