tbh not very much on a std car, depends on the body variation as well, mine being the smallest tunnel car being a base shell. if you never wanted to take the gearbox out without the engine then a very very small amount of tunnel work is req.
being a rally car, the necesity for emoval of the box is quite high, hence the large tunnel hole, not only that for me having had 2wd chassis mounts , i had to cut one off and move it, make a new mount due to steering rack position !!

other than that at std height , nothing else