I have used the 45/70 for quite a while...i currently have it in a 14" SSK barrel, a 21" carbine barrel, and a Ruger #1.
last month is used the SSK to take elk & buffalo, couldnt ask for better results. The guide was also impressed, especially when the elk went straight to the ground & never left his tracks.
I had a 16" taperd barrel TC barrel, and a 16" TC bull, and a 12" TC hunter all in 45/70 also, the 16's didnt balance quite right for me, & the 12 was a bit short for my taste.
whatever length you get, make sure there is a brake on it. The heavy loads were tolerable, but having fired 45/70's without a brake it does help. i dislike the blast from guns with a muzzle brake but i also dislike guns that kick so hard i avoid shooting them...
Factory 300 grain loads are quite good, and work well on deer, hogs, etc...for big stuff i like the 350 grain hornady bullet in my handloads. That bullet shot through the elk after breaking the shoulders. I did get the one back from the buffalo, it was just under the hide on the offside.
i am going hunting for big stuff again in a few months, Asiatic Water Buffalo are next, havent yet decided if i'm using the 45/70 or maybe a 375JDJ, will post some pix of whatever i shoot along with gun info, but chances are it'll be the 45/70 with 400 grain hard cast loads.
if you arent going to shoot anything bigger than deer there are a lot of other calibers that work fine without the recoil, but if elk, moose, etc are on the menu then its hard to go wrong with the 45/70 if you arent shooting much past a hundred yards or so...
hope this info helps some...
here's a pic of the hunt:
http://www.graybeardoutdoors.com/phpbb2/album_pic.php?pic_id=1060&sid=13d581421c2ac6cd034151bf3a25817f