Welcome to the forum!
Not a crazy idea at all, many have done it, and the results are very nice, though it seems more common to use a Volvo TD engine to repower a NA D24 LT van than the other way around.
A couple of notes: if you are planning to start with a 200-series Volvo, then you will want to find a M46 gearbox out of a diesel 200 series, not a 740/760. The mounting angle of the engine is different in the two cars; 700 series sits close to vertical, while 200 series is at a steeper slant. Any Volvo D24 bellhousing from a 200 or a 700 will bolt up to the ACL block, but you need the right one for the right car otherwise you'll have a lot more trouble making it all fit together.
The oil pan from the LT motor probably will not clear the crossmember in the 240, since the 240 D24 pan is very shallow (except for large sump at the back) and clearance to the crossmember and steering rack is tight even under the best of circumstances. I haven't seen an LT oil pan but Pinzgauer pans are quite a bit deeper overall and I believe they may share the same pan or similar. I would not be optimistic about being able to get the motor in the car with the LT pan on. You might also have ground clearance concerns if the sump hangs down below the crossmember. Probably better off finding a 240 pan and using it instead of trying to shoehorn it in with the LT pan attached.
Other things you will need, for the most straightforward install:
-motor mount brackets (on engine and crossmember) for diesel 240
-240 diesel accessory brackets and accessories (power steering pump/hoses, alternator, radiator fan unless you plan to use electric, etc)
-glow controller and wiring
-cooling system parts (radiator, hoses, etc)
-etc
The easiest way to do all this will be to try and find an intact non-turbo *diesel* 240 to start with, rather than a gas car. If you start with a gasser then you will have to either chase down a lot of diesel parts, or fabricate your own alternatives, which is always more work (and less clean) than installing factory parts already made for the same purpose.
Starting with a complete diesel car will provide you with the mounting bits, oil pan, gearbox, and ancillary parts that you need for a clean swap, and in addition the electrical and fuel systems already in the car will be ready to go as well. Just transfer the relevant parts from the NA Volvo motor to the LT TD motor, and bolt it in.
Re: rear end: The M46 has an especially short first gear (over 4:1) so you can use almost whatever rearend ratio you want, the taller the better IMHO unless you live in an area with very steep hills or plan on doing any towing or drag racing. Stock turbodiesel/M46 cars have a 3.54:1 ring & pinion which works fine. I think a 3.31:1 out of a NA M46/M47 gas car would be better in some ways. One benefit of the short low gear plus the diesel's torque characteristics, in the stock setup, is that clutches in D24 and D24T Volvos last almost indefinitely -- last one I took out had 330,000 miles on it and looked like it had at least that many miles left to give. There is also the aspect of transmission longevity; the M46 is not the most robust unit, so if you plan on adding power then using shorter rearend gearing may help the fragile trans hold up better. However, if you're aiming at building something for use as a daily driver and relaxed highway cruiser, I think the taller gearing will be a plus.
If you have already found a health TD engine then you have done the hardest part; finding a solid diesel 240 with engine problems should be easy!
Keep us posted.