The pistons and the engines are the same, the 3.9 and the 4.0 are the same, it was just LR rebadging. Obvious differences in injection systems excluded. You can check the pistons physically for stamped numbers.
As far as I remember, there is only one piston listed for both engines in Microcat, which I think I posted here in answer to a previous query.
I haven't checked bearing compatibility.
Peter