I've seen a P38 with a HMMWV Cummins and it retained the 4WD. IIRC, it was using the standard P38 drive train. And I recall a range of P38s that came with a 6.5l LS motor in them. If this old bus can take that sort of power, I'd be surprised if a Defender couldn't.I think the LS fits in but you lose the 4WD as nothing downstream can take the grunt.
there's a video somewhere of a Defender with a truck Cummins engine in. Same issue I think. A 2WD defender? Like a turbo on a push-bike.