The dimension on the piston from the centerline of the wrist pin to the piston crown is called "compression height". To change from a 5.7" to a 6" rod without changing the rod throw on the crank, you must decrease the compression height by 0.300". That means not only buying new 6" rods, but also new pistons. In a paper written by Ron Iskendarian, he states that the horsepower return for this change will be negligible, on the order of 5 to 7 horsepower. If every other area of the motor has been optimized for maximum output and this is the only stumbling-block standing between you and a national record, it might be worth the investment. If this is for a run-of-the-mill street/strip motor, there are many other ways to spend your money more effectively.