You could trim the the overhang shorter.

Use brackets to put the gutter in the right place. It's a good idea to separate the gutter from the roof edge anyway. It sounds like a half round would not have enough capacity so you might be stuck with a box gutter. The first step is to calculate the volume of water instead of guessing.

They make gutter hangers that attach to the surface of the roof. They are either screwed on with washer-head screws, or the nails are soldered-over (or capped by soldered-on strips of metal). These only work where there is wood sheathing under the metal, and from your question its not perfectly clear that this is so.
Casey

Hanging gutters are generally half-round and hold less water for a given width than box/K gutters. Check the volume needed first.