sstock,
In its original application the T5 was used with a side mount engine. In that three point mounting configuration the frame could flex without putting tension on the aluminum transmission case. Using a T5 behind a 216, 235, etc without a transmission cross-member allows the transmission to move free of the frame. Adding a transmission cross-member creates a rigid four point mounting configuration that transfers frame flex to the weakest point - the aluminum trans case. These old frames flex a lot.
I don't mean to argue against the use of a cross-member, but this makes the most sense to me and I test this theory on a daily basis.
