The 150mm of travel that you are requiring is for the frame. A rear shock can have a different stroke to generate the travel you wish.
What frame is it for?
I mean 150mm i2i, not travel. Is for a Wild Hare, to run in the 4.5 slot instead of a 165mm i2i, which is apparently possible, has been done and results in 3 and a bit travel, a lower bb and slacker head........think 4X Hare