ok im thinking of putting 600 engine in my 400 apart from engine carbs clocks and ecu is there anything else that needs to be done as havent got a clue lol

Do it..

But use the search function - hybrid is a good start.

I had one, raced it, it was wicked.

You need the downpipes.
If it's a 3TJ then you'll need a pair of mounts for the top of the engine
If it'a a 1WG you won't need them and it's a much easier swap.