1967 Leaf springs

I have a 67 Cougar . I am getting the springs redone at shop and they want to know what preload is in distance I am presuming . He rattled off 3 numbers that it could be according to his book . My car is a basic 289 4V vin number 7F91A512668 . I have owned 3 of these cars in the past am each one always appeared to be sagging on the ass . This one is stock with 4, 2.5 inch wide leafs on each side . Any suggestions . Add an extra leaf , an overload leaf perhaps

The spring shops that I have used asked only what ride height I wanted in relation to what the springs had. I would say, for example that I wanted the car to ride 2" higher in the rear. They would make the springs work that way. I have never added or subtracted a leaf or wanted any different spring rate or load carrying capacity.

I think your shop is asking you what load carrying capacity you want rather than asking what ride height you need.