You only need to modify the one rail. I'm not sure what they're trying to do by grinding the other one. If you enlarge the detents like the guy shows on the range rail, you should get L-N-H on both shafts without being able to put one shaft in high and the other in low simultaneously.
Chevy cases might be different, but I don't see any reason why. Either way, you can trouble shoot it on the bench. The lockout mechanism is stupid simple.