I have done shallow frost protected footings around the Twin Cities, 45 degrees North latitude. This project would be at 48 degrees North where the recommended minimum footing depth is 60". I believe all I have to do is have a wider "umbrella" of insulation, say 4'. I would not be doing a floating slab, just a shallower footing on heavy clay. However 5' of below grade foundation would help anchor this cabin in the top of a hillside. Opinions?

Some frost protected designs rely on building heat and others don't. For a cabin, I'd definitely want the latter. The width and thickness of the insulation varies with climate and it's best if you can put foam under the footing (otherwise the concrete can conduct heat down to the soil under the footing and freeze it). You might want to post a drawing.

Brad - assuming you are going down 18" or so, you might consider adding a few screw piles to the footings to anchor it.
Interesting that they call for 60" of frost protection on the 48th. We only need 48" on the 53rd. Frost penetrates on a 45º angle so if you need 60" and your foam was on the surface, you would have to go out 60", right?

Jon - If there is a possibility of frost under the footings, wether or not they are on foam, the design has failed and you could get frost heave. But I still agree with foam under the footings.

There are detailed guidelines for this published:

www.huduser.org/Publications/PDF/FPSFguide.pdf

I think there's a newer version of that. But you just follow the specifics for your climate. And as jonr suggests, it would wise to follow the more stringent guidelines for an unheated building.