Mehul, I am glad you got around to shooting that thing.
As I remember, I once had a Leupold variX3 on it and could easily hit beer cans at 200yds with it.
I thought of making it a 450 Marlin and also thought simply using 450 basic brass trimmed to the maximum length with whatever bullet you prefer would be pretty good since it would be as easy as rechambering to whatever length that ends up being. Maybe like a 45/90 or so with a 450gr bullet or a 458WM rimmed.