I think you are looking for the J P Gemmer conversion of the Remington Rolling block.
He "hawkenized" a variety of military rifles among them the Rolling Block, Trapdoor and the Sharps.
Also did caliber conversion and made several 40-65 trapdoors.I have some links to pics, will post when I can find them.
One of my next projects will be a Gemmer Rolling Block, 45-70 or 50-70, can't decide on the cartridge.
Wide butt plate, rounded hawken triggerguard, pewter endcap, thimbles, wiping rod, the whole 9 yards.
I have most of the parts in the shop, just don't have the time component.