Bilberries

My sister is a fan of bilberries, and I have promised her I'd see if we could grow any here for her. She lives in a condominium, while we are on 11 acres of mostly forested mountain side. We are technically in zone 6, but for practical purposes, I am more comfortable with calling it zone 5. We have not had very cold winters the three winters we have been here, but I have heard tales of 20 degrees below. We are on the south facing side of a relatively steep mountain at roughly 2200 feet elevation. Our soil is acid and grows a nice crop of wild blueberries, lowbush (very low bush!), in spite of the fact that it is very poor and they are mostly in the shade of relatively young oaks and black gums (it was clear cut about 30 years ago, as near as we can make out). Our soil is also very, very rocky, mostly shale. And we have lots of deer and, of course, lots of birds. Could we grow bilberries, and, if so, what preparations need be made in the way of holes – width, depth, replacement soil after we sift out all the rocks, which will leave us with very little soil?

The climatic zone you are in will be perfect for bilberries. Also, since your soil is acid and supports blueberries, bilberries should be no problem provided you can keep the soil moist enough.
It might be a good idea if you were to bring in some acid soil for the proposed planting sites, if the original soil is mostly rocks. Adding lots of peat moss to the planting hole would help to keep it moist and acid. A mulch after planting would also keep the moisture in.
For a planting hole I would suggest a hole with a one meter diameter and half a meter deep as a minimum.
Bilberries are slow to raise from seeds. They can take 4-5 years to reach fruiting stage from seeds. Growing bilberries from seeds is a challenge even to experienced growers, so please be prepared to allow both time and effort to grow a successful crop from seeds.