I say just go for the 450 Marlin. It's going to work great for what you want. Recoil really won't be that bad. I ran a 45-70 in a 10" BFR and shot nothing but Ruger #1 loads in it. It was a handful but hardly earth shattering.
Brass for the 450 is available and a little over $1 each. Factory ammo is available as well as published, tested load data. For bullets you can try running the 300gr TTSX designed for the 458 Socom. They're hard to get but should work pretty well for what you want.
Andrew
Brass for the 450 is available and a little over $1 each. Factory ammo is available as well as published, tested load data. For bullets you can try running the 300gr TTSX designed for the 458 Socom. They're hard to get but should work pretty well for what you want.
Andrew