Said that he'd gone through this before and after trying MANY different model springs, he found that the springs from a 420SEL did the trick. Apparently these springs are a little stiffer.
I trust this guy as well as anyone could trust any great Indy but I always like to get input from others on this forum.
Other things that I've read that can help raise the rear end is replacing rear subframe mounts and the diff. mount. As far as I know, mine are originals and should prolly be replaced regardless.