Congrats on your purchase. IMO the EL was one of the best configurations for hunting that Winchester released.
I’ve slugged a few Miroku 45/70 barrels and they came out at 0.457” - a bit tighter than Marlins tend to be.
I’d be more concerned about the bullet diameter ahead of the crimp groove and whether the bullet design has a front drive band. A thin band may work but probably best to have none at all and a bore riding design. The throating is non-existent as per SAAMI spec. In my Browning 1886, which also has the same throat treatment, I can load and cycle lead pills with an OAL of 2.8”, but to do that I need to use a bore riding design. A Lee FCD will help you fit a bullet to the throat if they are not quite right.
