I've heard differing opinions on all this, but have not seen definitive input for 4th and 5th gen springs on 3rd gens.
I know that 07-09 FJ Cruiser Springs will fit on both a 3rd and 4th gen, but I've read that 4th gen springs will not fit on a 3rd gen. Makes no sense.
I read somewhere that the latest model (2014?) FJ Cruiser springs will fit 4th and 5th gen (which in my opinion means they should theoretically fit the 3rd gen, as the 07-09 springs fit both the 3rd and 4th gen. Theoretically on top of this, this means that the 5th gen should fit the 3rd gen
I don't think the front shocks ever worked (maybe the rear shocks)but the springs are a possibility.
4th and 5th gen springs on a 3rd gen would be a good thing IMO because they are designed for vehicles that weigh up to 900lbs more stock than our 3rd gens do. So you should gain a slight lift if not more, and even if not, they won't be sagging
anytime soon.
I'd say try it out, especially since you have the parts free. I wouldn't pay money to test this, but a few hours of your time will give you your answer. At least see if the springs will work. I bet a lot of people would start to get 5th gen springs if you could confirm this. Especially in the future as there are more and more 5th gens in junkyards. People could get used stockish height 'firm' springs without the relatively high price of OME.
I was looking for a trashed 07-09 FJ Cruiser, but eventually said screw it and got some cheap Moog springs.
As long as the ID and 'tail' on either end of the spring match close enough , they should work pretty good. I know the 07-09 FJ Cruiser springs are considerably larger but work well still.