for the power steering box you count the bolts on the top 4,5,6, for the transfer box the numbers are on the back have a look at ashcrofts site theres is pictures and codes listed again its the ration rather than the number you are looking for 2.5/2.5td are differnt from tdi which are different from disco although all wil fit.
i have a disco transfer box in my 300tdi 110, slows it down slightly but is much nicer and quieter to drive, have a read around the web about this