I load .40 all the time with Longshot, but I haven't loaded anything in the 170 or 180 gr. range. Where does your 8.0 gr. charge fall into the load range, low, mid, or upper end? If it's the lower end I would move to about mid range data for that 170 and work up form there. If it is the upper end just load that 8.0 gr. charge for the 170, if it is working well on a 180 it will be fine for a 170 to start with. A rule of thumb is that when decreasing bullet weights the powder charge goes heavier, and the opposite applies for heavier bullets.
How do ya like that Longshot for .40? I use it for 9mm, also with really good velocities and accuracy.