One size spare tire for 2 tire sizes (275/65R18 Winter and 275/70R18 All-Terrain)?

For my 2009 Tundra CrewMax SR5 TRD Off-Road, stock height with new shocks, struts and springs, I have a set of stock size LT275/65R18 studded winter tires I just bought this winter and will likely last a few more winters. They are on their own wheels (used aluminum 5 spoke Tundra wheels). I'm about to replace my All-Terrain tires, currently the stock size, on the stock Tundra TRD Off-Road wheels but some of the new tires I was looking at don't come in that size. I could go up to a 275/70R18 which many people say can be done with no mods, or maybe remove/mod the front mud flaps, and it would give me a half inch lift.

The problem is... what size spare tire to use? If I get a flat tire, would it be a really bad thing to temporarily run a 275/65R18 tire on a rear wheel if the other one is a 275/70R18? I don't want to damage the gearing. But I also think of cars with the donut spare tires, often drastically smaller, but obviously temporary use and 55mph max speed or whatever the max speed is.

I have two steel wheels for spare tires, which I'm sandblasting and repainting right now along with my aluminum wheels for the soon to be purchased All-Terrain (or possibly Mud-Terrain tires). The original spare tires were old and cracked, so I got rid of them. My plan was to re-use two of the better Toyo Open Country AT2 in size 275/65R18 that are coming off my truck, to use as spares. I "think" it will fit in the spare tire well - roughly the same diameter as the 255/70R18 spare though a bit wider. I've heard of people putting this size and even a bit larger for a spare in the stock location. But, that's another story.

According to tiresize.com assuming all tires run true to size:

255/70R18: 32.1" tall, 100.7" diameter, 629 revs per mile (spare tire size)
275/65R18: 32.1" tall, 100.7" diameter, 629 revs per mile (stock tire size)
275/70R17: 33.2" tall, 104.1" diameter, 609 revs per mile (proposed new tire size)

The spare tire size and stock tire size are identical in terms of height and diameter, just not width. The easiest and safest option is to just buy tires in the stock size. But that would rule out a few of my tire choices which are not available in the stock size, but are available in 275/70R18. Otherwise, I thought since I have two steel wheels for spare tires, I could run one of each size on each of those steel wheels. Then in the winter, when I do my tire changeover, change the spare tire too. But that's a pain in the @ss that adds complexity. Or if I get a flat on a rear wheel while using 275/70R18 tires and I only have the shorter height spare, move one of the front wheels with the 275/70R18 to the back and put the spare on the front and be sure to run in 2WD. Again a pain in the @ss because then you change two tires instead of one.

Would be kind of nice to have two spare tires in the same size though, for simplicity. I hope to one day drive to Alaska, and they say you should always carry two spare tires, and probably best if all tires are the same size, I would think? Or is a minor variation ok? Or should I just shop for tires that are available in my stock size? Kind of takes away some of the fun and I won't get my half inch lift. Is it a big deal?