I presume that you've Googled and not found anybody else who's done this? Even if a thousand people have, it's quite likely that you'll be spending £500 at the very least on the special parts. If it's the D24T engine you're thinking of, it isn't particularly powerful, so I'd reckon that your £500+ would be better spent fettling your V8, or going for an LR diesel. You might also bear in mind that a straight six will probably longer than a V8, so you'll be moving the gearbox back, altering propshafts etc..
In my case it seemed that there isn't anyone who's fitted a Jag V12 engine to an R380, so I've had to machine the gearbox shaft, make a new flywheel and fabricate a bellhousing. That was over £600, and it's only the start. Standard wiring won't work, standard plumbing won't either, exhausts, propshafts, oil cooling, all different.
Don't want to put you off, but it is a big commitment.