I have the stock '74 4-sp Saginaw in it connected to the 4.3 Vortec. I am learning the differences in the T-5, T-50, and the World Class T-5. I am not anxious in having to move the shift lever at all if I change to one of these transmissions. The rear end is the 3:73 7.5in. from the Cosworth. In 73 or 74 they anticipated the Wankel rotary and made the transmission tunnel larger.
Build project can be seen on H-body.org website, Forums/Project builds. I think the Crossfires are in one of the pics in the garage.
Thanks for sharing your Vega pic.