Howdy, 1Glock!
For your Marlin 1895 Guide Gun, I'd load the same as I do for my 1895 w/22" Ballard-type rifling.
The standard factory 405 grain jacketed softpoint loaded to Trapdoor Springfield pressures and velocities will kill all the game named at effective .45-70 hunting distances. Cast or swaged lead bullets will perform nearly as well because of the pinky finger-sized bullet.
I would go with the 405 grains for more penetration, especially in the heavy-muscled elk and hogs you hunt.
YES, I know, the 300 and 350 grain bullets can be hot-rodded to close to 2000 fps using maximum loads. But, have you TRIED firing those loads? They kick like CRAZY in your GG and the muzzle blast is particularly fierce especially while hunting (no ear protection).
Oh, yeah. Ask around about the infamous muzzle blast and recoil.
The limiting factor on heavier bullets in your GG as it is on the standard M1895 is cartridge overall length. It will not feed, and will not eject loaded rounds over maximum length. That includes many or most of the 500 grain cast bullet loads and may apply to 405 grain bullets seated far out. I got tired or unscrewing and dropping the lever to clear action james the FIRST time in the woods.
HTH
John