Possum,
That soft roofing lead is perfect for muzzleloader and shotgun slugs. You can go ahead and mix it with linotype or other alloy to harden it, but it may have more value to someone who shoots muzzleloaders. You might be able to swap it off to a muzzleloader shooter for some harder alloy.
It's easy to harden up an alloy, but real difficult to soften it without adding pure lead like the roofing sheathing.