ARE the motor mounts on an LS1 and an LT1 the same?

No, the motor mounts on an LS1 and an LT1 are not the same. The LS1 uses a different mounting system compared to the LT1, reflecting the design and engineering changes made in the transition from the LT1 to the LS series engines. If swapping between these engines, you'll need to use the appropriate motor mounts designed for each engine type.