Adjusting The Height Of Front And Rear

The drilling of holes 1 inch lower is known as the "Shelby drop" and was done to early Shelbys for racing/handling purposes.
You'll want to buy a template to do it properly. Nothing worse than getting one hole off location a little. If you are just going to be doing normal driving then its not really necessary but then 90 oercent of the mods we all do here are probly unnecessary too I have the 620 springs that are also a 1 inch drop and I love them (also did the Shelby drop). If your coils are old and the insulators are worn out you may not get a full inch drop cause your old springs and insulators are drooping from age and wear.

http://www.stangnet.com/mustang-for...-classic-mustang-currently-sits.873021/page-4

http://www.stangnet.com/mustang-forums/threads/whats-a-good-interior-for-my-66-red-2-2.888407/

I have been on this path with my 68 GT 350. Before you start cutting or drilling, if you haven't solved this issue allow me to tell about what I experienced and did. First off, those rear leaf springs get tired after 20 or 30 years or more. And the leafs hold up the rear end. New leaf springs in the rear. I put mid eye leafs which dropped my rear end a full inch. I also did 4.5 leafs to make it more stiff. Next thing I did was rebuild the entire front end. John at Open Tracker Racing suggested I go with performance coil springs which dropped the car an inch. Although in my case the rear end turned out to be an inch lower than the front end. Kind a looked like your car. I called John at OTR and he suggested cutting half a coil off the front springs. My local tire shop does that kind of work so I talked to the owner and he said bring it out, they do that kind of cutting often. So he cut half a coil off and it dropped the front an inch. Easy and cost effective solution. Except I had 245 60R15's on all four wheels. These tires are an inch wider than stock. So the front tires rubbed the fender the first time I put it on the road to a car show. Took it back to the shop and he swapped out my NEW 245 60's in the front with 215/65R15's. This size is real close to the stock tire size. An inch narrower and 1/2 an inch smaller diameter than the 245's. So what did I end up with? First off the suspension is real tight. Handles unbelievably well now. Those 215 65's do not rub the front fenders. The car looks great lowered like it is. Last week I had the same shop install a set of Wilwood 4 piston discs and some Porterfield racing shoes in the rear. This car handles and stops far better than stock.

I happened across a conversation on this topic recently. The one author said the Shelby drop is for handling and does not lower the car like you are wanting to do. You need to use performance coil springs that are shorter. That is what hold the car up in the front. Same on the rear leaf springs. Mid eye shackles lower the back end a good inch. I might add that lowering my car gives it a pretty attractive stance. Plus I have inch wider tires in the back than the front. It's real subtle but attractive. People who have seen my car since the change have all made great comments about how aggressive & attractive my Mustang is. One friend said that is what a muscle car should look like. Not that I didn't get good comments before, but it's different now with the lowered suspension.