Reddit mentions of Nestle Nido Instant Dry Whole Milk, 12.6oz - Pack of 2 Cans

Get a variety, you'll need it. You can't eat the same thing every day or it will get really depressing. Stock up on all the foods you normally eat, rotate them (FIFO, first in first out, use the oldest first). Keep the shelves full and you will rest easier. Get some canned meat. Spam, roast beef, tuna, as the freezer probably won't be working. Anything you normally keep in the fridge or freezer find a dry, canned or packaged substitute that can be kept on a shelf. Get some powdered whole milk, most grocery stores carry it. Get snacks, jolly ranchers or hard candies are best as it won't melt and are easy to carry. It will bring you comfort in times of great peril. Also good for bartering. Get a couple 25 lb bags of sugar, good for a multitude of uses including dressing wounds, making booze and wonderful on cereal. Coffee will help keep you awake on guard duty, also great for bartering. Honey will last forever, get a bunch. The plastic buckets that cat litter comes in are wonderful for storing sugar, flour and other bulk dry ingredients. Just rinse them out, it's only clay.

Pulling a major change in your diet during a very stressful time will be very hard on your body. Don't do it. Prepare and make the shift as gradual as possible.

Unless you have the equipment to process wheat or oats don't even bother stocking up with a 50 gal barrel of the rough stuff.