A C6Z on DR's has run 10.9x, a C6Z on stock tires will run low mid 11's in good weather. One of the guys I race with has a C6Z and he is usually a 1/10th or 2 faster than me when he's on street tires and I will run mid-high 11's on DR's

 

I have West Coast Cylinder Heads stage 1 LS1 heads and a forged bottom end and my car is tuned to redline at 6750 rpms. The car still made power at 6900.

The OP needs to clarify whether or not its the shift light that goes off at 6k or the if he hits the rev limiter.

The light may be set low by the orginal owner to comply with softer driving habits. Take her out and get on her and see where the limiter kicks in or bring it to a tuner and have it run on the dyno again to find out for sure.

 

How many miles on the build? From what I've been able to gather there are a lot of variables in relation to how high is safe to spin. The valve train of course, light weight valves will help. Bigger valves will not, more weight to move. Springs? Was the bottom end touched? The rod bolts on early LS1's are a big weak point from what I've read. Not sure that a stroker can spin as high as a normal stroke, the reason being piston speed. The longer travel will cause the piston to be going a lot faster at both ends of the stroke, so more stress on the engine. I also think cam design would have an impact as well, as well as how long you plan to spend at the high revs. I.e. drag race or road race. High lift could hurt, but not sure about that. I'm no expert on the subject.

Another thing I've heard is that with a bigger cam the valve springs have a short life. If you got miles on the engine, you won't be able to safely spin it as high anymore. The higher performance you get out of a engine, the shorter the life of some of the parts. And, if you race then everything is a consumable

These are just some of the issues I've read about, don't have a whole lot of experience so I could be wrong.

So, its hard for anyone to say exactly how high you can go. If the engine was fresh then maybe you could be safe up to 6,500, maybe, but it would be better to have it evaluated by someone who knows LSx engines before spinning it up high.