Re: Need help with hidden gutters!
Hopefully they were fabricated from terne or copper, because you need to be able to solder the sections together for a proper job.
Box gutters start with the drawings, because there are limits to how much fall a given run can have; can't fall more than the thickness of the fascia, minus a couple of inches for framing + sheathing. Stuff like that. You can't extend existing runs, usually, because the outlet is already at the lowest point. I have never seen a run of box gutter over 48', and that was on a very large cornice, which you may even call monumental.
But once it has been worked out on paper, yes, building it is not hard.
Small puddles (1/8" of water over a foot or 18") owing to the metal not laying absolutely flat are not a defect, just the nature of the beast.
Remove not the ancient landmark, which your fathers have set.