I have a take-off trigger guard and backstrap from a Hy Hunter single action and a take-off grip frame from a Ruger Blackhawk. (The Hy Hunter screw pattern is the same as the Uberti) I compared these, did not put calipers on them, just eyeball side by side.
1. The lugs, as I said, are a little short, build up and dress down.
2. The trigger guard's front strap will require two bosses drilled for a pin to retain the trigger return spring, this just behind the cylinder frame.
3. A single lug is needed high up on the backstrap for a press fit pin. This pin hold up the rear legs of the trigger return spring.
4. A lug is required low on the front strap part of the trigger guard to seat the mainspring.
If you are all that handy, you may want to re-configure the mainspring strut. Uberti's web sight has an illustration of their new Horseman single action showing the lug and mainspring arrangement. (The Horseman is a new Uberti with transfer bar and coil mainspring.)
This sounds like an awful lot of work, but if you have access to the tools and equipment to do this, I believe you would be able to dope it out without too much problem.
Another thing, are we talking steel, or brass?