I would look at Lyman mold #429667 240 grain RNFP.
Allox bullet lube.
Harder isn't necessarily better.
A properly sized bullet that fits your bore still needs to be soft enough to bump up and make a perfect gas seal when it is fired.
I'd try 10 pounds of wheel weights with a couple ounces of tin bar solder added.
You can try that, or water quench them out of the mold if you still think harder is needed.
I can't recommend a load.
But those bullets should be able to be driven from 1,200 to 1,500 FPS without leading.
I would strongly suggest you buy the Lyman#49 reloading manual & the Lyman Cast Bullet Handbook before you even consider casting and shooting lead bullets.
Those are the best source out there for lead bullet info & load data for every caliber.
BNTW: If you have an older Marlin with Micro-Groove rifling, all bets are off.
You can get them to shoot cast bullets, or so I hear.
But it won't necessarily be easy.
rc