Gear box and transfer box

Sorry if this question has been asked before, I have a 1985 90 with a petrol engine and an LT77 gearbox, I'm thinking of changing the engine to a 200TDi for obvious reasons. Is the gearbox and transfer box I have ok or is there a better combination for more efficient driving on motorways etc?
Thank you for any advice.

will fit with no problem , might pay to renew the clutch aswell while there apart !!

You'll need to stick with your existing gearbox but you can fit a disco transfer box to raise the gearing a bit.