Stupid idea?

Hi I have look a little on gearbox ratio. How would it be if I take a gearbox from P38 and put it in my defender td5. Both is R380 but different gear ratio.

The engine rpm will go down to around 1800 rpm in 90km/h now its around 2050 rpm. I have 285/75-16 tires and alot of power at 90km/h. PSI-box, bigger intercooler, and little bigger turbo is mounted in defender.

I know it will be little slower but there is so much more power in td5 then bmw engine. Axle ratio is the same. I drive much caravan/ heavy trailer and have done the whit my P38 dse to. It was hard to start and then it was fine to drive.

I have a P38DSE that not start and Im thinking how it would be to have an Td5 and P38 gearbox.

Or I have also an extra Td5 engine in my garage whit electric box. If I change engine in P38 to Td5.

My Td5 I dont drive so much offroad and when I do it its not so difficult driving.

I drive much and like defender but sometimes Im so tired of all the noices. I drive around 700km every week. So im thinking of the fuel economy to.

Now I dont know which car I will do the change on, how would you do?

higher top speed, slower acceleration?

its possible ,but means changing input shaft ,mainshaft ,rear extension housing ,selector shaft, and differences are very subtle ,ive built a few td5 or 300 tdi boxes into p38 form ,no one noticed