I've got an older TRW coil book from my parts days in the 80's
68 GTO stock replacement rear springs are CC509. Wagon springs are CC507. CC501 shows up in 1970. They all will fit in the car. Here's a comparison of the specs:
CC501 ride height: 8", free height: 12.83", load: 750#, spring rate: 155#/in
CC507 ride height: 10", free height: 15", load: 850#, spring rate: 170#/in
CC509 ride height: 8", free height: 13.23", load: 810#, spring rate: 155#/in
Therefore, 501's aren't any taller than the stock 509 stance. the free height on the cargo coils means their shorter than the stock linear rate springs when you stand them side by side, but once the weight of the car is down on top of them, they should sit at the same height because they're stiffer. This can cause problems if the shocks have too much travel because I've had springs shift off the mounts when the ass end is unweighted enough (like flying over a raised railroad crossing like them Duke boys!)
Anyways, the wagon springs will suck. They'll jack the ass up two inches and it'll ride like a truck with that stiffer spring rate so even though it doesn't seem like much with only going two inches, I don't think you'll be happy with the ride.
Going through the ratings matching the diameters and end point styles I think a good candidate for a slight lift would be a CS5413 ride height: 8.5", free height: 12.93", load 740#, spring rate 167 #/in. I'm almost positive that's the spring I ran in my 69 vert. Napa still carried them 4 years ago when I put the car together, not sure if they're still available. it's got the slight rake I was looking for.
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