Nice! A few others of us on here have considered this swap as well. The electric OD on the M46 can be fussy for sure so the simplicity of a conventional overdrive 5th gear on the M47 is appealing. Plus the M47 is far lighter and smaller than the M46 due to the big OD unit hung off the back, saves a little weight and easier to handle during clutch replacements, etc.
I highly doubt the difference in strength could matter much even behind a D24T unless it were in the hands of an abusive operator. And behind a NA D24 it should be completely fine since the later year 240 diesels without turbo did come with M47s (late 80s when they went into the NA gassers also). To my knowledge the M47 never went behind a TD from the factory but that would likely not stop some of us with 7 series cars from trying it someday if the parts materialized.
As for swapping it for an M46, believe there would only be a few keys to success:
- Need to swap the diesel M46 bellhousing, possibly the clutch release fork, and definitely the sleeve on the end of the input shaft over to the gas M47. That sleeve is important since the gas engines use a pilot bearing with a smaller ID than the diesels. So if you forget to swap the sleeve you will have no support on the trans input shaft and it'll beat the trans bearings up quick. If this doesn't make sense the way I'm describing it don't worry, you will see what I mean as soon as you get the diesel trans out of the car and compare the input shaft to your M47. It should pull right off of there with a standard puller, maybe a little mild heat to help.
- Shift linkage might require some careful comparisons but if your M47 came from a 240 I would imagine it would drop right in.
- Driveline: all you need is the front half of the driveshaft from an M47 car and it should all go together with no fuss and no custom driveshaft work except maybe balancing it if you're using a front half from a different car and the rear half you already have. Note that you will actually be lengthening not shortening the driveline overall. The front shaft is longer on M47 since the M47 trans itself is considerably shorter than the M46. The connection at the trans might also be different since the diesels all always used flat flanges but the gas cars often used rubber guibos so that may be what your trans is set up for.
Finding a M47 front driveline section from a 240 should not be that hard? Any manual trans gas engine 240 from 1987 to 1993 would have what you need. Try asking on Turbobricks, those are pretty common cars and someone might have a parts donor sitting around.
Let us know how it goes, after having M46 OD relays and 4th gear switches fail more than once on long trips in the middle of nowhere and having to hotwire to get home, I would consider the M47 an upgrade.
Just fewer things to screw you up.