Yes you will need a conversion plate (pop up on e-bay every now and again). Engine mounts will need moving. Flywheel will need drilling for the Series clutch, there are bulkhead modifications and an exhaust will need making up too. You may also have to change to timing cover and water pump to get a 'short nose' one so that you can retain the recess grill; otherwise you can try and find an old 6-pot gearbox. They have a shorter bell housing and input shaft so that the old 6-pot engine fitted in the LR engine bay. Don't forget the brakes too, most upgrade to the 11 inch LWB setup on the front.
Not easy, do-able cos it's been done a lot and great fun afterwards. You'll have to make sure that you box is upto scratch if you have a heavy right foot and some people find that they can blow Series diff fairly easily too. :)
TBH, I'd either go for a 2.5 in good tune will a few ACR mods or go and get a V8 90 :D