I run them under my 55.
asside from the obvious things like relocating the hangers.
they are narower so ya have to boodie up a spacer. the bushings on the chevy springs have that steel inner sleve. Im not familliar with the diam of fj60 spring pins but I know the fixed pin on a 55 is way bigger than the ID of the bushing sleve. a custom shackle could easily adapt that side but you may have to come up with something for the fixed side.
SUA? unless your totally weighted down, you will more than likely get a bit of a lift out of them , even if they are stock.
and,..... run the overloads. at the very least, run minitruck overloads.
hope that helps a little :beer: