Damn--sorry to say this, but they are right. Anything larger than, well, the cam listed in my sig is too large for a 5.3L. I am pretty much at the WAY upper end for the cam size for a 5.3L. I only am running a 2600 stall cause I wanted to keep the driveability and I am planning on a Radix soon. Even a 6.0L would have a decently hard time chugging around with a cam that big. It isn't the end of the world--just make sure you get that stall and intake. Also, get some long tube headers and a 3" offroad Y-pipe. Since it is slightly exhaust biased, you need a good, free-flowing exhaust system. Oh, and most importantly the tune. That thing probably won't even idle on it's own without a tune. You don't have to sell it--go ahead and get everything in and see how you like it...