The D24T engine from a 740 TD can be installed pretty easily into a 200 series car IF that 200 car is already a diesel. But if you are starting with a gas 240, then you would really need a diesel 240 parts donor in order to bolt in any D24 or D24T engine. The 200 and 700/900 chassis are very different and share almost no parts: the 700 series mounts, driveline, crossmembers, etc are all incompatible with 240. Bellhousing is also different since the diesel engine mounts at a more vertical angle in the 7 vs the 2.
In short, gas 240 chassis + diesel 740 drivetrain parts won't get you enough to make a car bolt together. In order to bridge the differences, you would need many parts from a 240 diesel also. Preferably a complete donor but at minimum:
- 240 D24 oil pan, pickup tube, and dipstick
- 240 D24 bellhousing or complete M46
- 240 D24 engine mounts
- various fuel system, cooling system, cooling fan and shroud, electrical system parts
- edit: also need the 240 D24 style power steering pump bracket (740 style is stronger BUT interferes with path of upper rad hose so you need the 240 type), and air cleaner box and ducting if you want to use factory looking parts for that, and the coolant reservoir and windshield washer fluid reservoir and their mounting brackets/plumbing/wiring since those items are on the other side IIRC in a gas 240.
The trans crossmember and mount is the same as on the gas cars. This is true for both 2 and 7 series.
I think your Plan A of continuing to keep eyes out for a diesel 240 in the junkyard is best bet. If your plan is to swap into a gasser, then pull EVERYTHING complete from the car -- engine with trans, plus driveline, everything from the cooling system (240 diesel rads are NLA, rare to find used, and a gasser one won't be big enough!), and as much of the fuel system and electrical glow plug system as you can manage. Whatever extra all that stuff costs you to buy from the JY will pay you back many times over in making it easier to get your swap put together and sorted out.
Good luck!
Gas to diesel swaps are fun, definitely some work, but not bad if you are organized and diligent about the details to ensure a well finished end product. It can be a great way to get both the car and the engine you want, if they didn't come paired that way naturally. For example turbodiesel swap into a later era 1990s 240 would be a nice move, as you can then get features like ABS, better interior materials, better HVAC system, non-crumbling wiring harnesses, and other updates that were never seen in the early '80s model years that the diesel option was limited to in North America.