Read through a 15 page thread on a B234R conversion on MIGweb... various oil issues, partly due to mounting the engine vertically and partly from custom sump... not a straight forward swap either, although the chap doing it did seem to make it awkward by trying to stuff a Monza diff into an Omega casing, along with a Jag gearbox... whole car was basically built from scratch, and ended up being scrapped...
If it's the same Serek, he knows the thread as he posted on it
To anv6, if you want a 3.0 manual, then this is chance to get best of both worlds... find a scruffy post '98 3.0 manual, and pull the engine, box and clutch from it. Then fit 2.6/3.2 manifolds, 3.0 cats, replace all the sensors with the 3.2 items, and fit the loom from a 2.6 manual along with the coil packs, and plug this into the 3.2 ecu. Job jobbed. This assumes that the ecu is still with the car... otherwise you'll need the ecu, immobiliser ring and key chips too...
End result would see the engine and box you want in a newer Omega. Reading through... this was pretty much what you were told when you first asked...