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The 620's are front coil springs with a 1 inch drop. The options you have for rear leaf springs that I have seen are;
stock 4 leaf stock heighth(these will vary dending on the type of car)
stock 4 leaf with 1 to 1 1/2 inches taller heighth, ok why? LOL
heavy duty extra leaf(5) stock heighth
heavy duty extra leaf(5) mid eye up to 1 inch drop depending on the car

Just for grins I looked in my NPD catalog for your car, 68 vert, V8. They have a fairly compelete selection of springs now. You can't just get somebody's generic spring and expect it to be right. Well, heres their list;
68 289 vert - 5560-27
68 289 vert GT, 390 and 390 GT - 5560-29
68 289, 390, 428 vert GT comp handling - 5560-25

I've seen reverse eyes but these drop the rear end a lot. Maybe guys with tubbed rear ends or low riders do this or have their springs de-arched.

I priced having my springs re-arched with an extra leaf added. They add the extra leaf to make up for the re-arched weak old rusted spring. It came out to about the same that a pair of new mid eye 5 leaf springs cost. I'm installing them myself so I save some on the labor.

My philosophy on springs is that the front and back need to be balanced. In other words, its not good to put a higher rate spring on the front without addressing the rear. Otherwise, you can get into these situations where the car feels odd like its too stiff in the front and too mushy in the back(buckboard effect on bumps). That causes some really strange problems in handling too where the front doesn't roll as much but the rear rolls too much. The same goes for ride heighth in general although I like to lower the front just a bit more than the back on my car to get some rake angle for drag racing

My understanding on re-arching is that its somewhat of a bandaid. Since the metal is ancient and fatigued you're just re-arching old fatigued metal. It will sag again and this time at an accelerated rate. Well thats what I've been told. Maybe some of the metallurgists on here know more or better.

You can see my 65 fastback at:
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