Lowering

How to lower my 66 mustang using stock suspension setup  

I bought some shorter front springs from Laurel Mountain Mustang. Probably available elsewhere. No cutting and quicker fit, but make sure the spring rate is correct.  Be very careful removing old springs-use a spring compressor.  

If you do the '65 shelby A-arm relocation mod which requires two holes to be drilled in your shock tower per side you can use everything stock you have now (no new parts) and drop the front end about an inch. I've done it several times and it works great. The car handles better too. You will have to realign the car once you are done but that's about it.
For the rear end just put in one inch lowering blocks between the axle housing and leaf springs. The blocks are cheap and can be purchased at any auto parts store.

With both the easy mods mentioned above you will effectively lower your car one inch all around and still retain good ride quality and gain better handling for only a few bucks. It is also super easy to return everything back to the way it was if you ever want to for some reason.  

This website has alot of information on mustang suspensions:
http://dazed.home.bresnan.net/dazecars.html