RIP, When I saw those pix, I knew where you had landed. And with several buddies I see - good show!
Bugger asked ",So how close to 458 Win Mag could you get a 45-90 in a strong rifle (modern 1885)? I think mine had a 28" barrel" IMHO, close enough to kill the largest DG! At least that is what my Miroku 1886 .45-90 with 26 inch barrel did with 450 grain NF, Punch, and Kodiak at 2150 fps. An ele, multiple buff, and a BIG old leopard.(300 grain Nosler PP at 2200 fps). These were all taken within 100 yards range, some much closer where you get to see the dust fly when the bullet hits.
You could likely tweak out a higher velocity, but it is really not needed for hunting + the added velocity might increase recoil.
CRS, NRA Benefactor Life Member, Whittington Center, TSRA, DWWC, DRSS Android Reloading Ballistics App at http://www.xplat.net/