eaton has springs that go 2" under stock for the rear. ( i have them on my 66. )Here is my car 5 months ago when I started to restore it...you can see the OEM height was pretty low for the 20" wheels the previous owner had
Now, deducting 1" from this height should be enough for me, nice stance but not too low
I could have gone 2" inches, but was afraid of rubbing
As for the rears, I havent got to that part of the restoration, but might stick to just 1/2 inch drop or 1"
Eaton Detroit can do 1-2 inches in the front but just 1/2 in the rears....
Dearborn classics have these which are custom, and can go from 2-3 inches depending on your liking
1965-1972 Galaxie Custom Rear Coil Springs 2-3 Inch Drop - Ford Parts
Personally, I like the front to be lower than the back.....I would do measurements first, to make sure everything will clear your wheels, etc
Then, visualize how your choice in drop will affect the look of the car