I have 158 grain LSWC bullets in .358" diameter. Are they hard enough for good 357 Mag loads? What is considered hard cast? What velocity would you recommend as max before needing Gas Checks?

'hard cast' is anything containing antimony.

15 BHN is about 4 more than I use.

I got nuthin on needing gas checks I have 1 [one] mold that uses them and I generally use it in my 445 super mag.

if I had to guess at a speed I'd say 1400 fps. is about right.
but I've gone to 1650 in my lever rifles, and could probably go past that with H-110.

I shoot them at 1330fps out of a 6 inch 686

Without a problem

With cast bullets “it depends”. Size is probably most important and if you have more than one .357 to feed you may need different sizes for optimal accuracy.

In my experience gas checks help with fudging sizes or high velocity. I normally use half(ish) clip on wheel weights with half(ish) pure(ish) lead, water dropped for everything from mild .38 Special to .358 Winchester loads over 2,400fps.

I couldn’t tell you what the hardness is of my bullets, remember they need to obturate enough to fit the bore and too hard is not a good thing.