I would not bother with a .45 Colt cylinder. I would use .454 brass and load heavy .45 Colt type loads in it as Tradmark suggested.
I have owned a 7-1/2" (scoped with a 4X Leupold) FA, as well 6" and 4-3/4" versions, and liked them all. I took the most game with the 7-1/2", followed by the 6". I never hunted with the 4-3/4" version, but it was extremely accurate, and I liked it. I used only full- strength loads in the 7-1/2", and somewhat lesser .454 loads in the others.
Barrel length preference is such a subjective thing, that I'm reluctant to make a flat "best" statement. I think you should try the gun as-is before making any changes. Shoot it, wear it, perhaps hunt with it, then decide.