Air shocks are bad because they unload the rear springs (can you say wheel hop?) and raise the center of gravity. Longer shackles are bad because they overload the rear half of the spring.
If you have problems with the wheel wells hitting the tires then your tires are too wide or your wheel backspacing is wrong. Air shocks and shackles are just VERY poor bandaids.
Thanks for the advice. Just trying to get some FAT tires under the car without affecting the ride height...much. Stock on my Boss 302 were F 60's and when I saw they were reproducing the L's I thought maybe......
Okay, no air shocks. The closests I can come to in metric is a P275 60R 15 with a tread width of 8.7". The old L60 Polyglas GT has a tread width 9" and a section width of about 11.4 on an 8" rim. I would never consider shackels, but how about spacer blocks on the axel perches?....that is if there is any chance of the tires fitting. I'm not looking to raise the car in the back any more than necessary.
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.