MBeige: I probably didn't explain my question very well, so let me re-do it. The bottom seat in the 240D is in great shape, nice and firm; all I need to do is remove it, replace the MBTex cover with my Bamboo cover and install it in my car. I wouldn't have to mess with cushions or anything else like that. So, given this, will the seat bolt up in my car without any problems? Thanks.
You might run into some problems with mounting. As I recall the early models had a different mounting setup. Since there is a "facelift" in terms of interior design pre-80 to post-80, the less items you swap over, the less problems you'll encounter.
If you'll remove the bottom cushion from the 240D, and swap over your MB-Tex cover on it, you'll be fine but you'll now have to consider if the seat frame will bolt on to your older model.
If you'll remove the foam from the 240D and transplant only the foam into your 300D, you'll have less problems to worry about.
You'll be halfway there the moment you replace the MB-Tex anyway, all you need to do is remove the hog rings on the horsehair padding and remove and vacuum the horsehair out of the frame, replace it with the foam pad (if it does have a foam pad), replace the hog rings and then install the original upholstery. No need to worry about re-dying.
You can inspect the 240D from behind the front seat to check if it has the orange foam pad. Getting the foam pad also allows you with more options, because the front passenger seat foam pad should be the same as the driver seat foam pad.
Worst case scenario, grab the bottom half of the 240D seat, and try your method first. If the frame does not bolt up, you can use my method and still manage to fix your seat problem.