Under-geared defender 110 with newish 300tdi

Just put a 300tdi into a 1997 MOD Canadian Defender pickup. Now I have lots of power but gearing is to low for highway comfort. I want to but larger dia/circumference tires to lower RPM at highway speeds. looking for input. Presently running LT265/75R16. Is this practical?

does it have a R380 gearbox or a LT77?

Lt77

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.

If it's ex-military it might have the lower geared transfer box with a ratio of 1.667:1, rather than the usual 1.4:1. If you want to go even higher, you could fit one out of a Discovery which are typically 1.22:1.

Rougharse racing just beat me to it.

Then it's probably got a low ratio transfer box. You can swap it for a disco box to give you more speed but might be worth going the whole hog and getting a r380 and disco box

Oh, I probably should have looked up and saw that some clever chaps have already answered

Thanks for the suggestions and info.....don't want to spend any MORE money soon as wife has sharp knife.