Cast alloy for HP bullets

I started out casting boolts for target shooting from Lyman #2 , but more at time goes on I am looking at hunting with cast bullets, I got a .460/405gr HP mould for my Marlin 45-70 and ordered a .454/255gr HP mould for my 454 Cassull .
I have started casting for the 45-70 with SOWW giving me an 8Bhn air cooled bullet, I am out of SOWW and have pure lead coming.
I am looking at pushing both of these bullets from 1400 to 1700 FPS.
I want them to expand doing plenty of damage but hold together enough to do push there way through on deer , pig or black bear.
20 to 1 alloy with 10Bhn does not look hard to get or to make, not much harder than SOWW’
Am I on the right track to get these bullet to function to what I am after? Do I need to think about going harder?