LS1 crankshaft end play should measure 0.0015–0.0078 inch. Learn where the No. 3 thrust bearing is located, how crankshaft end play is measured, and what to check during an LS1 engine build.
The OBDII trouble code P1121 is due to your throttle position sensor signal being lower than expected by your ECU. Problems that arise from this OBDII...