Hi and welcome to LOC sorry for the late reply been on vacation.
The instrument housing will need to be for a series 2 (1993-1994) LS400 as the 1990-1992 are different but before you replace it check the speedo pickup on the transmission as this may be the problem.
If it is the instrument pod there are many repairers who may be able to sort your own pod have a look on Ebay.
thanks