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.

Geez beattie,you now better than to listen to old wives tales but seriously be advised that longer rear shackles will also effect rear roll steer(or is this what your aiming at)as well as lift the rear.
This is one of my clues to making old Dattos go round corners.

I can't imagine why longer shackles would make the leaves sag since they are not bearing any addidional weight & for normal street duty, the springs wouldn't flex any more.
In bush bash mode, the greater clearance that the axle has to the bump stop might allow a greater degree of flex & perhaps it's this that results in sag, but it wouldn't be the shackles per se'

If bottoming out is the problem then long shackles only address the symptom, not the actual problem & the fix is very likely going to involve a chat with a suspension specialist & some re-arched springs, possibly with a stronger spring rate if bush bashing is what the car is used for. Perhaps some good shocks too.

Their called shackles and yeah make some yourself. Just get some flat bar and drill holes in it. Would be easy as for small shackles like this.

But going on info from the 4wd world and from personal exp they make leaves sag and soften 2ce as fast! But also give better flex

make some from scratch

need a set to lengthen to raise car, can anyone help. cheers