P38 6 Cylinder Diesel in a Series

Hello. Before you all think, WHY??, its because i want to and i just need some advice. Thankyou. I have a very early complete Range Rover P38 DSE Manual truck and a Series Land Rover and a Stage 1 front end. I have read as much as i can and my biggest stumbling block is mating the defender R380 gearbox to the BMW 6 cylinder which already sports an R380 manual box but the transfer box is ass about face. I understand the Series axle thing and the CV joint versus UJs. I have early Discovery 10 spline diffs to up the gearing and im a competent engineer/welder/fabricator. I want to use the Defender box as the knobs are in better positions. Anyway, back to the gearbox issue. Is there a chance that as there is already a manual box fitted i can get away with a transfer box swap. No negativity please. At least not too much. I love the sound of the 6 cylinder engine ever since i used to drive the old International 6 cyl on the farm. It wont be a daily driver, just a Sunday cruiser. Its a later Series 3 so the brakes are already 11" drums which work perfectly well when they are up to scratch. Hope someone can help me. I have viewed the othert hreads available thanks.
Many thanks
Terry