Did this ever go past the research stage? I have picked up an 1998 Mercedes W210 E300TD with the OM606 and 722.6 automatic gearbox. I picked it up on the cheap while I fix my daily, and I've decided I'd like to build up the engine to swap into something call. Having always wanted a P38, I think this is a potentially good candidate.
To do this, diesel pump UK sells adaptor plates to match the R380 transmission to the OM606.
174BHP is not the limit of the engine, as with a 7mm mechanical fuel pump built up from the W124 and with a hybrid turbo, reliable figures in the 400HP range can be achieved.
Some hiccups I can see being had are P38 being more electrically involved than a defender. This swap has been tried and tested with the defender, but I've only seen a short video from Finland on this swap being done on a P38. As mentioned before, the sump clearing
My aim is to verify this is feasible for the average have a go hero, then to buy a relatively alright P38, 4.0 petrol or 2.5 diesel and R380 convert it (based on thin pickings of factory manuals I can find on ebay - manual as standard of course being prefered). Meanwhile, I'll build up my E-class with the tastier turbo and pump. Once both are up and running, will try frankensteining the two together. I can't see this being done for much change from 10k including cost of the RR without anything major going wrong, but I also think that it'll make for a pretty capable and reliable car at the end.
Would be amazing if anyone has done any research on this since, particularly given how much easier DieselPumpsUK (not an ad!) appear to have made things like this in the last few years