I've just purchased a 82 single cab 4wd Hilux which has been converted to Rover v8 power

It still has the original 4spd g-box connected using an adaptor of unknown origin. Due to not having a 5th gear it tends to consume petrol pretty rapidly with motorway driving. Does anybody know whether a later 5spd gearbox would be a major swap? Would the current 4spd adaptor fit the bellhousing of a 5spd or are they a very different in bolt pattern?
I've done a little research and found that the 82 Hilux did come with a 5spd, but this was in the USA. I've considered bigger tyres or "taller diff's but this would be relatively expensive compared to another g-box..
Has anyone done this conversion.?