I run the 55 springs out back only. And I did not flip the shackle hangers. All I did was get 2.5" extended shackles. They make a very nice 45deg angle this way...Originally posted by woody:
Running the FJ55 rear packs in the rear of your 40 will lengthen your rear drivetrain by 3.5" and require you to "spin" the shackle mount around.
Switching from your current FJ40 front packs to FJ55 REAR packs will move your axle 3.5" back, shortening your front driveline. Final result will be a 93.5" wheelbase.
Keeping your current front springs and exchanging the rear springs for the 55 packs will net you a 97" wheel base. (stock is 90", reversing front springs = +3.5" and reversing rear springs = +3.5")