45-70 loaded with the Core-Bon 350 gr load. Even the 22" barreled Marlin is a pretty good handling gun. Guide gun is just too short IMO.
The only suitable bullet for handloaders that will work through the lever action repeater is the 400 gr Speer and its too soft, or so they claim, for big bears. Most shotgun slugs are too. A handloaded hardened round ball is a better load for a shotgun.
Speer makes a 350gr 45 cal bullet that is hard enough but the bullet is designed for the 458 and will not feed though the Marlin seated to the cannelure.
Second choice would be a hard cast bullet of about 400 gr. at 1750fps or so. RCBS makes a GC bullet that would work OK for this.

Dan