You probably need to talk to Kert. I don't think there's a "best length", I think the theory of the bar length is going to want to place it's front mount as close to the center of the vehicle as possible but that's probably not practical for you. You would also need to consider that at a certain length (??) the bar is going to want to bow under heavy power which would defeat it's purpose.
I also wonder if the carrier bearing placement will create a problem.