Re: Reslope hidden built-in gutter
The boxed gutters that I know of start out very shallow at the high end; 1.5" deep, and end up at 4 or 5 inches deep at the leader end. So to re-jigger your gutter, the maximum slope will be determined by how much depth you have at your shallow end.
I'm not hopeful that this can be done without taking it all apart and building new wooden brackets and redoing the sheathing, and relining with metal, with all the details that entail from it.
Remove not the ancient landmark, which your fathers have set.