If it started life as a 2.5 N/A diesel it will have a 1.6:1 transfer box, 300tdi Defenders were fitted with a 1.4:1 transfer box.
If you don't know the ratio of your transfer box it may be on a sticker on the rear face of the box, or google the prefix of the the serial number.
I fitted a 300tdi to my 1987 ex-MOD 110 and ran it for a short while with the original 1.6:1 transfer box and standard 750x16 tyres and found it massively under-geared.
It now has a 1.2:1 transfer box from a 300tdi Discovery, which transformed it, but would be too tall for oversize tyres.
If you go too big with the tyres you will have problems with them fouling the suspension arms and you will reduce the steering lock.