Actually, there are a number of "easy" options for FJ40 SOA projects. I chose to do mine with FJ55 rear spring packs at all 4 corners. These are 3.5" longer than FJ40 springs and have a centered pin. In the rear, your axle will move back 3.5" and you reverse the mounting for the shackle. In front, you can keep the stock axle location with a shackle reversal....the front mount stays in the same relative location, the shackle eye gets recessed into the frame 3.5" further back of it's stock location.
My front springs have 9 leaves, my rears 8. In front I use the entire travel of a 14" stroke Doestch Tech shock, so there is plenty of travel, even with that many leaves.
FJ60 rear springs will net you similar results, but hanger mods may be required since the spring eyes are even a bit larger than those for a 55.
I've got numerous writeups on my site on SOA projects for FJ40's...including mine.
