Hi Mike,
Well, you be talking about Apples, Oranges, Lemons and Limes. The Colt 1860 was made with Both Iron and Brass trigger guard/Back strap. The 1860 was also made with both Army Pattern and '51 Navy pattern Grip Sets. Colt would convert anything you sent them so long as it was mechanically sound. Bottom line here is any Colt pattern Grip Set can be "correct."
Now ... "a ton of fitting" or ..... One's definition of a ton of fitting is dependent. First and foremost, a Lustrum or two ago, Uberti "standardized" their frame dimensions, screw locations and screws. This means new build Uberti Grip Frames are interchangeable throughout the SA line. Example, a BP 1860 Grip Set will mount to an 1873 Mdl P. The actual fit will NOT be exact. Close, but not exact. Brass is much much much easier to fit than steel. Surface brass to dimension and polish .. done. Steel .. re-blue.
Then ... The search for your hearts' desire. VTI Gunparts. VTI carries "sets." The sets include Trigger Guard, Back Strap and finished wood, all of which have been fit to the factory "buck" and fit to each other. Not cheap but damn convenient. Highly recommended. I have used a LOT of them.
Bon Appetite