A 205 and a 275 have a pretty big difference in diameter, but the 70 series against the 60 series could reduce the size differences by a lot as well.
I think for a spare, I would see if I could split the difference, maybe a 225 70 spare would work out OK on both the front and the rear. Then plan on only running the spare long enough to get the correct size tire back on the car.

To put things in proper perspective, a lot of people only have those doughnut size spares and they seem like about 1/2 the diameter of the normal tires. Those they recommend less then 60 miles and under 50 mph. Not sure I'd want to do a sure grip 8 3/4 that far, but a couple miles might be OK.