A few years back, I had looked at the same thing; reboring a rolling block pistol to a centerfire approximation of the old 50 cal Navy Rimfire.
The difficulties I found were:
The Uberti RBP is scaled down from the old Remington action. One of the 60's NRA gunsmithing guides* shows the original barrel with a 1-inch diameter. I believe the Uberti barrel is 3/4-inch; not much meat left for a healthy 50 caliber cartridge.
*The Navy and Army Remington RB pistols were very popular for conversion to single-shot rifles back before the GCA of 1968. Nowdays, even if you could find one, you couldn't restore it to pistol configuration without running afoul of ATFE.
I had even gone so far as to see if that Texas RB action company would consider a run of full-scale pistol actions, but no interest.
So I bought a 45-70 barrel for my Contender, and I feel much better now.