There is a lot to explore here, do you have "THE BOOK"? If you have everything else sorted out, I would bet that the 50-110 would beat the snot out of you, and would be very hard to regulate. Is that gonna be fun??? On the other hand the 45-70 is always recommended for the easiest cartridge to work up loads for and will be fun to shoot. It also has the widest range of Bullet weights available. Also The 45-90 can be loaded down to the 45-70 velocities pretty easily, or load it up and you will have a wider range of "regulation" possibilities, I think. What is your goal? Are you Elephant hunting or just having fun?....NOTE, I have never built a double rifle or done a conversion,,,,,just my .02,,,,,LK