You should do a quick search for the answer to your question.
There was an excelent post with lots of links not long ago talking about the LS6 cam and push rod lenght. The answer is that you need longer pushrods, but I can't remember the specifics.
The only reason the LS6's use the 7.4 pushrods is becasue the valve stems are longer. Otherwise they would need longer pushrods also.
I also know several people using the stock pushrods with no problem. When I had the stock ones in I had some valve noise that drove me a little crazy.
Ross
No more trucks, but this is what I used to have. 2000 GMC Sierra, 6.0L-LQ4, Built 4L60, LS6 Cam, The Other Guys Headers, 3 inch exhaust and 7 chamber flowmaster, 43lb injectors, Nelson Tuning, Drop - 3/4 McGaughys Spindles, Bell Tech Shackles & Hangers. Best time 13.59 @ 106