I would go with a 4L65 or 4L70, which is the same transmission with more gears in the planetaries.
The L96 you are looking at has a separate transmission controller for the 6L90. All you would need to do is get a separate controller for the 4L70, and Chevy has built vehicles that way.
How are you going to use this car? If you're not going too wild, the 6L90 might be OK - but it sounds like that's not the plan.