what gearbox is better for p40 engine ,5 speed or 4 ?

do i have to change any thing so i can use the 5 speed transmission ?

I believe the tail/prop shaft lengths are different
A 5 speed will give you better highway driving

5 speed has overdrive, thats the only difference,except the SD 33 n/a and SD33 T has a different first gear
The 5 speed box is around 100 mm odd longer,so longer front shaft and shorter rear

I'm curious if I can manage an SD33 transmission swap on my 1969 KL60. Can someone tell me if the bell housings of the P and P40 are the same?