T-350's came with one of three different tailshaft housing lengths. Most have the 6" tailshaft housing, but most '73-up trucks use a 9" one, and many full-size passenger cars from the late '60s up used a 12" one. Here's a site with trans dimensions: http://tciauto.com/tech_info/trans_dims.htm
Watch out for bad bolt holes in the main case. The bolt holes where the rear trans mount attaches to the case are often broken completely out of the case. Might be fixable, but I bet it's a pain.