I'd imagine the P38 auto box will be a problem as it's electricery, you'd need a 300TDi with it's own auto to match up.
Not sure but I think the transfer boxs are interchangeable, you'd have to mess about tiwht engine mounts and exhaust but I'd imagine you'll have more space when you've finished than you have now.
Why the change though? I'd have thought a 300 would be gutless with a P38, they're heavy.