seeing as how i just researched this all yesterday, i will be happy to enlighten you to (what i believe) are correct answers.

the LS rods (PR4) are wider at the bearing end and the (wrist) pin end than the 'R' family of pistons are. therefore, if you wanted to put CTR or ITR or GSR pistons onto LS rods (which you need for the LS crank), you would have to remove some material either from the sides of the pin end of the rod, or from the inside of the base of the piston, to allow the pin end to fit into the piston with ample clearance.

there are (at least) two other options, however, and from what i understand, both the USDM (PR3) and JDM (P30) B16 pistons will fit onto LS rods without modification.

the USDM (PR3) and JDM (P30) pistons, on LS rods, will result in a C/R of somewhere around 11.3:1 and 11.5:1, respectively.

NOTE: if you were able to put the B16B pistons on LS rods, you would end up with a C/R of somewhere around 13:1, so you wouldnt want to do that anyway.