Howdy,
The real question here is whether you can safely shoot .454 bullets with your Marlin. Whether your pressures will get too high is dependent on type and size of powder load
and size and hardness of bullet,among other things,like crimp and case sizing. But,assuming your loading down for CAS,powder shouldn't be a problem,so bullet hardness might be the deciding factor. Slug your Marlin bore and see what size it is. You may have to just use two different size bullets.
Personally,I shoot .452,but I shoot 200 gr. bullets from my pistols and 250 gr. bullets from my rifles. I use two Uberti 1875 Remingtons and a Marlin Cowboy.
Bellicose Bill