Gio, I just put some meats under my 70 which should be the same as yours. Check out my recent picture posts. You will see it looks pretty darn god. Most ppeople just put 8" all the way around ant that's to wimpy in the rear. I put 9.5" in the rear and it fits just right. Check it out.
Ps. beware of the backspcing. I don't remember what I did in the back but if you are really interested, I can check it out for you.
I recently pulled this from Car Craft or Mustang & Ford (can't remember which) website. This was listed for Mutangs, but I believe Cougars are similar. I'll try to resurrect the location of this information.
15 Inch: 1969-1970 can use 15x9 with 5-1/2-inch backspacing 225/60 tires front and 235/60 tires in the rear.
16 Inch: 1969-1970 can use 16x9 with 5-1/2-inch backspacing 225/50 tires front and 245/50 tires on the rear.
17 Inch: 1969-1970 can use 17x9 with 5-1/2-inch backspacing 245/40 tires front and 275/40 tires on the rear.
18 Inch: 1969-1970 can use 18x9 with 5-1/2-inch backspacing 245/40 tires front and 265/35 tires on the rear.
I think the springs were originally Maier that I bought for my XR7G some 16 years ago. I took them off of the G before I sold the car. I think if you go to Mustangs Plus, they have the same ones available. They run about 1" lower and are rated heavier. Here is the deal, I put on a 1 1/8 sway bar with the springs and with the low profile tires, the ride is a little on the harsh side. I only drive the car once or twice a week so I can live with it. If this was a daily driver, no way. Good luck. Z