I've used 1680 with both a 357 Max and my 350 L. In my 2 examples I couldn't fit enough 1680 in either case with a 180 grain bullet to see any sort of pressure signs, at all, but easily exceeded 296 and Lil gun velocities; those two hit pressure issues well before a totally full case and 1680 velocities.
Anyway, I'd suggest starting at around 25 grains with a .355 180 (or your 170) and go up from there. My loads with 180s topped out over 30 grains and as said, compressed loads rather than any sort of pressure signs were the end point. I've found 1680 to give the highest velocity over everything else with 180+ bullets, but requires a lot more powder to get there compared to Lil gun,, 296, etc. Very accurate as well. Should work great with your 170s.
