Yeah, ballast lead is dead-soft, at least all the ballast lead I've seen is dead soft.
How fast do you intend to push those bullets? For target bullets and midrange stuff, simply adding tin in a 5% ratio will improve castability. Any more than that and you're wasting the tin.
I use very little linotype, mainly in rifle bullets where I'll be driving them over 1500 fps and need the toughness that linotype imparts to the alloy.
The one thing that will improve your bullets is the tin, for .357 velocities. If you want them a little harder you can water-drop them which will harden them. The one thing you'll want to pay attention to is sizing. Your bullets might drop from the mold and not need sizing, or they might need to be sized. You won't know until you have bullets in hand.
The best resource online for this type discussion is the
Cast Boolits forum. Lots of old hands over there willing to pass along good information. Go over there and read the stickys, then start asking questions.