If you want highest available velocity for the 170 with your existing powders, there is Hodgdon Lil Gun on-line data for your specific bullet. Be very careful with both case length and with COAL. Some folks have reported some pretty bad problems with Lil Gun that I do not think we fully understand. On the other hand, there are lots of folks who say they use this powder in the 350L with no problems. I you use it, be very meticulous for sticking with the recipe and be very observant for any signs of problems.
Using 2400 should get you acceptable performance with slightly less velocity for a given pressure. I know of no "unexplained problems" with this powder. H110/296 and Lil Gun are the powders I associate with what I call finicky performance. I have seen some 2400 loads for the 350L out on the web, but none with pressure data. There is 357 Max data for 2400 with pressure test data from the old Alliant guides. Working up using Max data should be safe. If you want to understand what to expect based on your barrel length, your COAL and your case capacity as compared to a published 357 Max recipe, I can run some Quickload stuff for you.
Unique will work for a plinker in the bolt gun. Actually, Titegroup is also pretty good for this type of thing. If you want help for this kind of load, more details on what you want to achieve would help.
https://shootersreference.com/reloadingdata/350-legend/
https://shootersreference.com/reload...a/357-maximum/
http://marvinstuart.com/firearm/Manu...20-%202005.pdf