On the subject of gearing, yes the original tyres are quite small and while this makes for good slow and hill work, it has the engine running at 3000rpm+ on the highway. At least on the petrol 4cylinder ones.
Hilux wheels/tyres are the answer, same stud pattern but bigger/wider. It raises the gearing quite a bit.
The problem then is power (or the lack of it) to push the higher gearing on the road.
I'm guessing the turbo diesels would handle it ok, but the standard 2L petrol doesn't like 5th gear unless its flat or downhill!

Phill is your Delica diesel or petrol?