What powders were you thinking of? I have 358 data from JD Jones....The data I got from him lists all sorts of different bullets, but does not say anything specific about cast bullets....however, at the top of the page he lists some 250 gr loads, that are lower velocity...but does not say what bullets they are.
47.0 grs of 748 @ 1861 fps
48.0 grs of 4064 @ 1959 fps
44.0 grs of H322 @ 1920 fps
46.0 grs of H4895 @ 1939 fps.
I think you are on the right track with starting loads for 358 Winchester, similar velocities.
Also..45.0 grs of AA2520 should give you just under 2,000 fps and
33.0 grs of AA5744 should be about the same.
But....you could E-mail JD and get it straight from the horses mouth. info@sskindustries.com
He is always pretty good about a personal reply, or (740) 264-0176, 264-7217