I cant say for sure. I had cut coils (1 coil removed) and the headers would clear most speed bumps. Then I lowered the a-arms, and I would scrape on anything. I put stock coils back in, and now I can drive over anything. Its lower than stock, but I couldnt tell you how much. I dont think its enough to worry about it (if you are).
The car sits on the spring (S), and it's only the shock tower side (T) that is getting lowered. The balljoint side stays the same. Since only the one side is getting lowered one inch, the car only drops about half that. Purchasing the negative wedge kits puts the balljoint at a better angle, making it wear better with this modification, and also allows the arm to be lowered a bit further without bind.
i took a look at that template, but it only include cars from 1965-1966 and 1967-1979, but i own a 1971 mustang, can i use this too or is there some other template for this ? or isn't this drop for me ?
Since most have already given you the tech info, I will tell you that it makes a huge difference in handling and in tire wear. It tracks better, it handles alot better and the ouside of the tire wears less. At a minimum get yourself a template a nice sharp center punch and a good sharp drill bit. The ball joint wedge kits are a nice product too and not too expensive as car parts go. Mustangs Plus and Sacramento Mustang sells them(probably the closest to you there in Chico).