The GM T5 housing is cast thicker in some important areas. Don't get my lying trying to tell you where.
I was told to acquire a GM T5 housing and put my G-force gears in it. I would have to do some machining to make the taller 5th gear to fit, but other than that... it will work.
I have not done this yet, so it's still heresay. I would think someone from Astro or a G-force tech would be able to answer this definitively.