one tip for using longer than stock shackles, taken from off road use.
after the length is decided upon, drill a hole in the middle of the shackle and add a spacer the width of the distance when the shackles are tightened. this will add strength, as well as some stability. this spacer can also be welded in place.
as has been already suggested, if one has some strap steel [1/4 x 1 1/2 or 2"] and a drill press, these can be easily fabricated to your liking.
however, one thing that hasn't been mentioned. after getting so long, these seem to impart extra stress on the rear of the spring, causing the main and second leaves to bend downward.
i saw this 60 years or so ago, and i see this today, when using super long shackles.