im pretty sure the ls pistons stay in the ls block
but i belive either one will work... the only difference would be the compression,
the gsr pistons have a higher compression than the ls pistons

also, i am not positive but the gsr pistons may be .25mm bigger than the stock ls pistons so if that was the case you would need ur block bored .25 over stock, making ur block fit 81.25mm rather than 81.00mm pistons... but like i said im not sure, i may be just getting them confused with type r pistons...