On my 67, the rear shackles, were at about 5:30 with the standard arc. 7 1/8" but had a 6 1/2" gap between tire and wheel well. I was told to de arc the springs. It gave me the ride height I wanted. But had to buy longer shackles and now shackle hang down and are at about 6:30 and hit the back valance. How can I lower the height and get back to the stock shackles? And what angle should the shackles be?
If you want to lower an inch you could use stock length shackles and use a 1 inch spacer between the leaf springs and perch on the rear end with longer U bolts. De arching springs flattens them and the shackles will hit lower valence
I would consider replacing the springs with some Detroit speed leaf springs that you can get with the correct length with a 2" or 3" drop. Extended shackles are a no no as it changes all of the rear dynamics resulting in more wheel hop, braking issues, etc… Plus they will strike the rear valence and tear it up and make a lot of noise.