T10 Issue
 

I had my Borg Warner Super T10 Rebuilt, I changed my case from a Aluminum to a Cast Iron due to BBC Torque, now I’m having a problem with Reverse, when ever I put it in Reverse it will grind out, I even tried going into 1st then Reverse and still have the problem and sometimes my shifter will get stuck in Reverse, it only grind out in reverse every other gear is fine and when i let my car roll in netural it doesn't grind

Check to make sure everything is properly adjusted. If its all good then you need to contact whoever rebuilt it for you, that's unacceptable to have a problem like that after being rebuilt.

Did you change the clutch too? Sounds like more of a clutch problem to me. If the clutch doesn't completely disengage then the transmission will grind when going into reverse. 1st through 4th have syncronizers to stop the gears before going in, but reverse doesn't have a syncronizer, so it grinds if anything in the transmission is moving.