Have you shot other (commercial) cast bullets from this gun? Do they lead?
In my experience, leading comes from either the barrel (some barrels just seem to lead up) the load, or the size of the bullet (too small or too big). Some say the lubricant can also have an effect, but I haven't experimented enough with lubes to say one way or another.
As for the hardness of the alloy, yes, I think it matters, but only in the sense that the size of the bullet and the load have to be matched to the alloy you use. I have never tested the hardness of my alloy (range scrap plus linotype). It is hard, but you can still mark it with a fingernail (that's my test). However, I also shoot swagged hollowbase wadcutters in my 38 Specials, and I don't get any leading. The swagged bullets are very very soft.
This is just a guess, but if you are getting leading the full length of the barrel, I would say that your bullet is undersized, and that its skidding over the rifling, A hot load would exacerbate the problem. Have you slugged the barrel?