Hello, new to this and need help

I recenty bought a 1981 series 3 landrover which someone has put a discovery 300tdi into. I think it has a lt77s gear box in it because of where reverse is (up and left). The problem is it only has a top speed of somewhere just past 50, would this be because it has the wrong transfer box or would it be because it has the original axles in with the wrong diffs? or maybe its neither of these?! just looking for some advice and info really. Thanks

changing the diffs would help,you can also change t/box ratio depending on what you have fitted, its usually better not to run in 4wd constantly either by fitting a kit in t/box center diff or free wheeling hubs