DSMcG
While mine isn't a Malcom, it is based on the same concept. A little background, first I called and inquired about mounting a Malcom or Dixie on my 38-55 Target rifle. It's the same as the BC but smaller caliber and 4" less barrel.
I was told, and I believe incorrectly now, that the Malcom or Dixie mounts would not work. Standing AS IS they will not mount, BUT if you are willing to cut a dovetail in the barrel, forward of the chamber area, it can be mounted. Be prepared to spend a good deal of money, as often the mounts are several hundred dollars themselves. And decide if you want front or rear windage mounts.
The Malcom, as I understand, has a small rib on the belly of the scope, that the mounts use to keep the crosshairs aligned, and for lighter recoiling rifles they do use spring mounts. I see some using a recoil spring, similar to Unertl type scope, by the way those type of mounts DO HOLD ZERO quite well. I also see Malcom clamp mounts meant for high recoiling rifles, which the 45-70 would be.
You'd have to get the scope, mounts and take them all to a smith, unless you have the capability yourself, and have them fitted to the barrel.
Mine is a slow work in progress and I have high hopes for when it's done, but time will tell. Mine is NOT the Malcom quality, it's like comparing Simmons scopes to Swarovski.
Keep us posted on what you decide and if you go ahead, any progress made. Good luck.