I'll add my 0.02 here. I load all of my 357 cases down to around 38 spl velocity. I don't mind shooting 38 cases in my 357 but I have so many 357 cases I just use those. Alliant has some load data for what they call "cowboy" loads that appear to be about 38 spl velocity or maybe a tad hotter but they work really well in 357 cases using 158 SWC bullets which is what I use.
Here's the link.
Alliant Powder - Reloader's Guide
I use Amarican Select but Unique would probably work just as well. I've never tried Unique so don't know. These are
max loads. Start 10% below listed load and work up. AS is an exceptionally clean burning powder. I use it for 38 spl, 9x19, 357 mag
lite and 45 ACP.
Good luck.