I have 285/75/16 cooper discoverer ST on my 90, the only thing i'v had to do was adjust the steering stops and thats with tired suspension, new susp in the pipe line.
the 285s gave me an extra 2" on the diff height.
I used solvent free (didn't fancy the resultant solvent head fook) evo stick contact adhesive, the material has a certain amount of 'spring' to it so this helps holds it in place.
Yeah it's your intake for the heater, you dont need to do anything with it, getting water in there wont do any damage.
If you do get a large amount of water in there there is a drain on the bottom to release it.