Double check the throttle linkage and be sure you are getting 100% throttle opening at the throttle body. An LS1 will spin the tires off the rims unless you are running 35" or taller tires or 3.08 gears behind an automatic. Also, are you sure you are getting the right amount of fuel pressure? If you're low on FP then you won't have the power. Do you have a catalytic convertor? If so, are you sure it isn't plugged up?