Yes the gearing would be different, often the ratios won't change a great deal 1-6 (perhaps not the case with a JTD) just the final drive ratio differs I don't have the numebrs for the ratios to hand, can often be found on Italia speed, the JTD setup on a nutter V6 like yours could be amusing, how fast would you have to shift through the box
70mph needs a snatch for 4th
much harder work to get anywhere than you petrol boys, you'd need a lightening arm with that on your car (or maybe not as you have more revs available), Thing is the gearbox would most likely just bolt on.