When I researched this issue for a rebuild of my G56 on my 3rd gen truck (2006) the recommendations from the forum were Blumenthal out of Oklahoma City:
http://www.blumenthalmfg.com/
Or Standard Transmission & Gear out of Ft Worth:
https://standardtransmission.com/contact.html
I went with Standard because of their reputation and better pricing. Standard also did two upgrades: (1) new front retainer - steel and (2) weld the reverse shift fork. I paid around $3500 vs Blumenthal which was over $5K. Oh and that included the oil. Somewhere along the rebuild path someone stripped the threads on my fill plug so now I have a leak I'm trying to solve. Many people including me could have been responsible for stripping the threads.
After I had already had my transmission rebuilt I ran across this Company selling new heavy duty G56 cases and transmissions on Ebay:
https://high-impact.net/product/heavy-duty-g56
I have not heard anything about High Impact so maybe others might have experience with them?