Yes, a same powder charge/bullet will have less pressure in a 357 case versus a 38 special case.
I wouldn't use starting 38 loads in a 357 case, otherwise I'd not be concerned. I don't know what powders you have on hand?
I would avoid 2400 for light loads, it works much better at higher pressure. With the 170ish Keith, I go with something along the lines of 5gr of Unique in a 38 special case and 6grs in a 357 case.
