3.5 V8 Gearbox

Hi, I have a 110 3.5 V8 Carb, this is a project, so far things are going fairly good. My gearbox is extremely grindy when I lift the clutch. It is a manual 5 speed LT85 I believe. I was at a breakers last week, he said that he had an autobox from a 2003 Discovery V8 that he believed would fit, he could supply everything needed. Could anybody please tell me, would it be worth it ( is it a really big horrible job? )or should I go down the exchange gearbox route. Many thanks