The 1872 OT was not a conversion. It was manufactured as a new design and not a conversion of an existing design.
My 5 1/2" .45 Colt 1872's are some of my favorite "gamer" guns for Cowboy Action shooting. When going "gamer", I shoot a .454" round ball over 6.1 grains Trail Boss for ~850 fps and very light recoil - 6.1 grains instead of 6.0 because that's what the corresponding RCBS Little Dandy rotor throws.
I seat the ball almost
flush with the case mouth (the rules say it can't be seated below the case mouth), and smear some solid stick lube grease around the ball/case mouth. It is an extremely accurate load in all three of mine and shoots to point of aim at CAS ranges. If you don't crimp and just leave the mouths belled (slightly enough to chamber, of course), the brass lasts a long time.