I really like the Marlin Guide Gun. It's a handy little carbine-length rifle that gets the job done.
It will never be what it wasn't designed to be - it's not a long range rifle. But it's a great short to medium range brush gun that will knock down any medium to large game within 150 yards. That's why mine wears a receiver-mounted peep instead of a scope.
With 45-70 all you're accomplishing with hot loads is making it more punishing to shoot and maybe a tiny bit flatter trajectory. I load mine to just over max for a trapdoor and starting pressure for my lever gun. Recoil is manageable and it will humanely kill anything in North America.