All Daihatsu's of that body shap have the same diff ratio. However there are several diffrent transfer box rastios. The lowest is the F80 (2Ltr petrol). This transfer will go straight on your existing gear box, or the whole gearbox/transfer asembly will marry up to your Deisel if you swap the bell housing and primary shaft. Also if your 75 is a Turbo Deisel, then a transfer from a normally asperated should be between the two. This will just drop right in, as it's basicly the same engin.
As for lowere diff ratios, I belive (though not 100% sure) that F20 (1600 petrol (hard-top's look like Postman Pat's van)) will go straight in.
If you want to be really silly, I belive you'll find that the F20 transfer is even lower than the F80, however this would take a lot of fiting. If you could find an F20 5 speed box/transfer that would fit if you change the bell housing only over. But youy would also need to change the prop shaft flanges, as these are diffrent sizes. This could be a good preecursor to fiddle brakes if you wanted them, as an F20 transfer has a drum hand break on the back, leaving your F75 wheel mounted hand breakes free for what ever you like!!! However be ware I have been told that an F20 transfer off of a 4 speed box will not marry to a 5 speed box, but I have not tryed that one myself.