I have to agree with azgn, if you're already running 6.8's at over 100 in the 8th, then your problem isn't fuel rail or fuel line related.
You're likely running out of injector. You would be much better off running 6 injectors properly sized for your performance level, rather than undersized injectors and a 7th. Trying to tune your present setup has to be problematic at best, simply because there's no way to monitor the 7th injector's operating parameters on any scan tools we have. It's a crap shoot at best.
Even if the 7th you have is huge, and running static, you're still under injectored.