I'm way, way behind you on the whole "whiteblock in a 240" track (hey, at least the engine is in the car), but that hasn't prevented me thinking about how I'd get a manual behind it, and from what I've read, there are some decent BMW-sourced options that we actually got here in North America.
See if my benchracing makes any sense:
I'm running a 225/50R16 on my car (24.9" diameter), the stock E90 335i runs a 225/40R18 (25.1") and the difference in tire diameter seems comparable enough to my admittedly ignorant self. The final drive from the 335i is a 3.08 (though the M3 runs the same gearbox with a 3.85). To the best of my knowledge, the highest ratio Volvo offered in the 240 was 3.31, but I believe you can get aftermarket Dana 35 3.08 sets as well. Plugging the GS6-53BZ gearbox ratios into here:
https://www.blocklayer.com/rpm-gear.aspx
With a 3.31, that'd put you at almost exactly 2500rpm @ 100kph in 6th gear, which seems pretty reasonable for highway speeds.
This option does, of course, require cutting and welding together the AW30-40 bellhousing to the BMW transmission, and I'm sure that's not trivial depending on the tools you have access to. But the overall cost would likely be well less than a CD009, it sounds like it fits without tunnel bashing and you can even buy a bolt-on transmission cross member from the Swedes:
https://www.anderssonsteelspeed.se/...alk-bmw-zf-6vxl-diesel---volvo-240?tm=webshop
No way it's as cheap as an M46, but at least you get the keep the sweet inline six you spent so much effort getting into your car.