No disrespect intended to your dad, but he needs to hold it tighter to the shoulder.
I've got a .450 Marlin. I think the .45/70 is a better choice if you're a handloader, just due to the cost of the .450 brass versus the cost of .45/70 brass. In a strong gun they can each be loaded to equal power levels.
I just "lucked" into a trade for a .450 so I settled for it instead of the .45/70.
I think a 325gr bullet is a bit light, given the intended purpose of the rifle... but if it brings in the meat that your dad is aiming at, then great. I favor 405gr hardcast, gaschecked bullets for mine.