advice on a 45 70 marlin guide gun vs xlr...

I have a guide gun set up with peep sites.
Shoot well, but never hunted with it.

Where I used to hunt back east all game was under 60 yards.
I live west now. The gun gun has not been given a chance.

I am thinking a xlr with the 24" barrel and a low power leupold scope.
I reload.

How much gain in velocity is there with the longer barrel?
How much better trajectory with warm loads?