The fronts are the same length as FJ40 springs; the rears are 47" long - using the FJ60 rears in the rear of an FJ40 puts the axle in about the same place as if you flipped the springs.
What you have to remember, or know, is that the FJ60 spring eyes are much larger than FJ40 spring eyes, and do not fit in the spring brackets. The FJ60 uses a much larger spring bushing, which makes the spring loop (?) that much larger.
55 packs likewise have a larger eye but it will squeeze into the stock FJ40 mounting location. Running FJ55 REAR packs all around would net you about 7" of increased wheelbase, 3.5 at each end, assuming you are not running a shackle reversal. You ought to be able to just flip the shackle hangers end for end and "bolt them up." Poly bushings with the correct/"close enuf" ID can be located at Spectre www.sor.com
55 front packs are same length as 40 series. Rear packs are 3.5" longer, and like 1/4" or something shorter than rear 60 packs....very minimal difference, but eye size is different.
Rear FJ55 packs are like hens teeth to locate. I run a stock straight-cut set with one extra leaf (9 total) in front and a stock tapered cut set (8 pack) in the rear. (Different year 55's used different rear leaf designs....some had all leaves with a straight cut end, some with a rounded and tapered end to most of the leaves. The rounded/tapered leaves will offer a softer ride, and are best used in the rear to equalize flex.)
Actually 60 series springs are two different lengths. The fronts are 1058 mm and the rears are 1160mm. 40s are 1070 mm front and rear. 55s are 1071mm front and 1155 rear. It is true that 60 use wider hangers, although I think they might be the same as the 79-84 40s. I'm not sure what all of these numbers would translate to for wheelbase as the center pins are not centered. So you may be able to gain a little or a lot. Flipping the springs and redrilling the military leaf may have the same net effect without all the hassle. I have some stock 60 springs in the shed if you would like me to measure center pin distances.
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