Overdrive units

[122s penni]

Given that you all seem to be so knowledgeable and helpful on here, I wondered if anyone can help me with another query.

My daily driver is a 122s with a B20 lump and 4 speed box. As I do a fair amount of miles for work, and rebuilding the SUs didn't seem to make much of a dent in the (quite hideous) fuel consumption, we sourced an earlier 120 series which has an overdrive box and the intention was to put that gearbox in the daily. However, the one we've picked up has a B18 engine in it and from what I've read, the B18 and B20 overdrive units were different (hydraulic pressure differs i think?).

Does anybody know whether it's still possible to mate the B18 box with overdrive to the b20 lump that's in my daily driver or is it a case of needing to swap the b20 and 4 speed for the b18 and 4 speed with overdrive i.e. complete transplant? (assuming possibly need to swap prop shafts over too) And would I really notice the difference between a well set up b18 and the current b20?

My other 122s is a b20 with overdrive (but don't really want to pull that apart as this is a customised one which is really starting to come together) and it went to beaulieu and back from Cornwall on 45 quids worth of fuel. However, after a mild improvement in consumption on the daily, it's back to returning about 25 to the gallon, 30 if I'm lucky (figuring perhaps my right foot is an issue or I'm paying the price of no overdrive?).

Any thoughts/advice would be gratefully received