The following is from another forum and might give some more background info.
:Late P4 110 3 litre and P5 3 litres | Rover P5 Club Forum
A 95 engine is virtually the same as that in a 110 (P4, not Landie
) except that the 110 has a Weskake head). The 6-cyl engines in the 95,100 and 110 are short-stroke versions of the 7-bearing engine introduced for the 3-litre; they're different to the earlier 6-cylinders in the 90, 75s and the 105.
The manual gearbox in the MK11 3-litres onwards has different ratios to the earlier 3-litre. If you know your engine number we can probably work out which model it was in originally. The comment above about crankshafts is correct in that Mk1 and MK11 auto cranks were different to the manual ones but MK111 were the same.
I've no idea how easy/difficult it is to do what you're considering (I've just re-read your original thread and realised that the 95 reference is a red herring but I've left my comments as originally typed!)
What age is the 90? Those up to1956 had free wheel, 56 onwards had overdrive as an option.