What do you call big-bore?
45-70 16" is going to give you enough, IF you load out your own, to take anything in North America....including Bison...you can load these up past the pressures listed for the old Trapdoors.
As far as the recoil...its not that its EASIER as a pistol...its just not hammering against my shoulder.
The Rynite shoulder stock is really small and narrow...almost like a stock from a Enfield JC...gives out more PSI...I am sure that with a decelerator or some such it would be more forgiving...
as far as barrel length...you can't beat the 14" length...It gives you plenty of barrel for good velocity and small enough to make it easy to tote around. They even make shoulder holsters for scoped TC's up to 14".
The ONLY drawback to these wonderful little toys is the scopes...pistol optics are generally harder to come by and a tad more expensive...I own Tasco, Burris, Weaver, Simmons, Leupold and TC factory (allegedly Burris)...and IMO the Weaver and Leupold are about the same in quality and fit/finish...
The best way to go however, no matter what scope, is to get at least two sets of rings, and place a third scope ring on whatever you mount. It will do more to let that scope live through the rather stiff recoil of the larger bore cartridges. The smaller bore cartridges you can go with two rings on a weaver base, TC Duo mount, or the Leupy dovetail mount...I have mostly weaver and the Leupy...no Duo ring...personal preference.
You will need to get at least one extra frame...that way your not constantly swapping barrels...then it grows...the sickness...welcome to your nightmare.
Also, just to add...just got me a jug of "cheerio's" to play with...I have a starting load of 12 grains behind a 400 gr. slug will do your 900 fps...I used to use Unique for my bunny-buster loads...but Unique makes my barrels heat up more quickly. YMMV