I am also a fan of "Hell on Wheels" so a Griswold replica would be way cool for me, but the stickie here warns against brass-framed revolvers. This makes sense to me, so I am probably dropping a Griswold, sadly, from consideration.
If you like the half round barrel of the Griswold & Gunnison, but are considering dropping that idea because of the brass frame, I would suggest you get a steel frame 1851 and then look on ebay for the half round Griswold barrel. The octagonal barrel 1851 and the half round barrel Griswold and Gunnison are the same revolver, except for the half round barrel of the Griswold. So you can get a steel frame 1851 and simply swap out the barrel for a half round Griswold. I'd suggest you make sure they are both from the same maker though, so that the pin holes will more likely align. However, you can file out the pin holes to make them fit if they are slightly off from the barrel being made by a different maker.
If the barrel to receiver pin holes are WAY off, another thing you can do is pull the pins out with vicegrips. Then beside the old pin holes (where the pins were before you pulled them out), you can drill new pin holes and use your old pins in them, or if you buggered up the pins by pulling them out with vicegrips, then you can use cut down and rounded end drill rod as pins and hammer tap them in. Then drill new holes that those pins will align with. That's worst case scenario though. More than likely the pins and holes will line up just fine, and if they are a little off, as I said, you can use a round jeweler's file to ream them out a little so the pins will align and fit into them.
Regarding the differences between the Uberti and Pietta Remington model 1858 revolvers, here is why I think the Uberti is better.
Although as Doc Hoy mentioned the Pietta's are doing better in their quality these days, I still think that overall in fit and finish
that the Uberti 1858 Remington's are just a leeetle better. I've owned both
, but sold my Pietta 1858 and kept my Uberti 1858. But the slightly better fit and finish is actually not the main reason I like the Uberti 1858 Remy's better.
The main reason I like the Uberti 1858 Remingtons better, is because the front sight and loading lever catch are dovetailed into the barrel, whereas on the Pietta's they are round ended and soldered onto the barrel, so they can't as easily be removed on the Pietta's as on the Uberti's by just tapping them out of their dovetails. Plus if a front sight or loading lever catch gets loose on a Pietta, you have to solder it back tight again to fix it. They don't get loose on the Uberti usually, and if they ever do, all you need to do is peen a bit on the edge of the dovetail to tighten them up again.
In order to remove the cylinder pin completely from the 1858 revolver, you have to remove the loading lever catch. And since the Pietta loading lever catch is round and soldered into the barrel, it isn't easy to remove. But the Uberti loading lever catch is easy to tap out of its dovetail in the barrel (as is its front sight). Also by being able to tap the dovetailed front sight sideways in its dovetail on the Uberti's, you can easily change the windage on the Uberti 1858 or easily change out the front sight completely for a taller or lower one to change your elevation. Can't do that on a Pietta 1858 since it's front sight can't be tapped sideways with a hammer to change windage because it is soldered into a hole and can't move from that hole.
That's the main reason I like the Uberti 1858 better over the Pietta 1858.