Right ive got some 265 75s bf ats for my disco what do i need to trim and do i really have to lift it? and if i do where can i buy a set-up tomorrow?

Hi guys after servicing my disco this evening ive found something thats worrying me at the leading edge of the bellhousing there is a threaded hole with nothing in it what is this? im not worried its a drain plug but i could be wrong but the box is a little overfull if anything cheers in advance

Im dom 24 from nr selby