The factory 405 grain ammo is OK, but loaded down to original pressures out of respect to the hundreds of older, black powder arms that remain in use.
Your nickel steel barrel itself is a special order item. Although Winchester used those barrels on all of the lightweight guns, 26" rapid taper barrels were not standard. Not having seen the gun, it's difficult to establish a dollar value. An educated guess, with a 50% finish factor, would be somewhere in the neighborhood of $1500.
The fact that collectors place a premium on the older 1886s is lost to me. I am quite the opposite, owning a fair number of '86 Winchesters, with all but one being post 1910 guns. I search out later guns with different calibers and features than standard guns would have. Contrary to what is generally accepted, Winchester continued to make guns of whatever caliber and configuration a customer desired.
You have a nice gun in your possession, one that still has the capability of making a fine hunting rifle. Handloading is the way to go. If you don't load, there are a number of companies that make high velocity .45-70 ammo.
:grin: