Putting blocks on the axle perches will lower the car. Its the opposite of a truck.
If you need to raise the back of the car, I would use the shackles from Mustangs Unlimited etc that have one additional hole. They are short enough to stay hidden behind the valance. That will raise the car about 1/2 or 5/8 of an inch.
It may just be the the springs on the car are old and have sagged some. If thats the case, new springs might be what you need.
I have seen 69-70 Mustangs with 255 60 15 on the rear, but I don't know about 275s.