1968 gt 350 stock heights

You are dealing with a lot of variables here. From the factory the ride heights varied. Typically a 67/68 Shelby sat low in the back (more so than a regular Mustang) because of the beefed up front springs. However, because of lack of spring quality control they did not all sit the same.

Even today spring rates are a crapshoot. You could buy two sets of the "same" spring and they would probably not sit at the same height. IMO a set of springs is just a starting point. I would buy a set that you expect to sit high (Mustang stock ride height) and then lower the car as desired by putting lowering blocks in the back and cutting the front coils. IMO you should not use lowering blocks more than 1" thick because any more than that leads to axle hop. And don't start with 620 lb front springs and cut them because you will end up with the ride of a gocart.

So if you get a set of rear springs and they sit 2" too high then get another that sits lower (like mid-eyes). I do recommend that you get 4.5 or 5 leaf springs to prevent wheel hop. Get a set of compromise front springs (like the 550 lb ones from Grab-a-track) figuring you will have to cut them 1/4 coil at a time until you get the height you want.

As an example: on my 68 Mustang vert with a 428CJ motor I put in a set of 1969 351 Mach one springs. It sat way up in the air - even with the heavy motor. I cut 1/2 coil off them to get them where I wanted. The half coil dropped the car 1.25" but because I started with a milder spring the ride did not become unbearable.

 

The reason they sit low in the rear is because of the competition suspension rear springs which are -1" from regular

Maybe if you use off shore stuff. I always use the Eaton springs because they are building them to the same specs as they did in 68 (original supplier) Never had an issue with how they sat. I don't think you want to be messing with cutting springs and adding blocks to the rear