Longer shackles are the way to go on 1200 track cars. Not sure if they are legal on the street.

As Bert has suggested, it will influence the roll centre of the rear. If you can position the front and rear eyes of the leaf spring the same distance from the road surface [leaf sring is parallel to the road], the car will load up and un load better through the corners. By having the eyes of the leaves at different heights from the road surface it will induce a twisting type effect as the suspension loads up around a corner.

If you are going to the effort of making new rear shackles i would make them adjustable, then when you adjust the ride height of the front you can optomise the rear to match.