I don't have a .450 Marlin, but I do have the 22" 1895 in .45-70 which is much the same.
The 405 grain loads at 1500 FPS are downright unpleasant to shoot from the bench, so I would not even want to try the 1800 FPS loads.
From the bench I find that the recoil drives the rifle back enough to open the lever (after the bullet has gone), so I use the overhand grip on the lever to prevent this.
The stock design was intended for the .30-30 class cartridges, although I find it quite comfortable even with the 1500 FPS .45-70 loads.
With a scope mounted, the comb will be too low on any Marlin lever gun.
And yes, it is a hell of a lot more pleasant to shoot offhand.
