Between DEfenders and Discos / RRs, the overall final drive ratio in HIGH RANGE is fixed by the Transfer box gear ratios.
Fitting Disco transfer gears to a Defender does what you say - poor man's overdrive..
Remember, doing this will up ALL the gears, including first and reverse, so starting off with a 3½ ton trailer won't be quite so easy.
Use low range to get it rolling then snick it into high range, and NEVER slip the clutch to get a big load moving.
Big truck trick is to use a very LOW gear from a standing start, let the engine idle, and on no more than idle speed ease up the clutch gently. The truck will move off, and you can let the clutch pedal right up. THEN you deck the GO pedal. Same in every gear, clutch pedal right up, THEN hit the gas. With 15 well-spaced gears it's a breeze.
LandRover clutches appreciate the same kindness.
CharlesY