I cast wheel weights as is. At moderate velocities, say under 1200 fps, they are hard enough. Above that, I drop them from the mould into a pail of water. My 45 Colt loads are mostly 650 - 800 fps or so. My 405 grain 45/70 bullets go into the water, even though I usually keep the velocity under 1350 or so. No problems with leading on either.
The 45/70 bullets hardly deform hitting the rock fines pile (tailings) and a bud of mine figured the water quenching likely gave them a hardness of about 20-22. They work well, and are great plinking bullets.
Gueßsing you are casting for old slab sides, so you velocity is likely under a grand. You should be good to go.
