Hi, brother. Thanks for the kind words of appreciation.
Actually, when it comes to such modifications, NOTHING is impossible...so any technician who says that this-or-the-other cannot be done, is taking the easy way out.
Have you considered mounting your FJ40 body atop the 70 Series Chassis? That way, you get the chassis, suspension, and disc brakes from the 70 Series, whilst you keep the beautiful body shape of the 40 Series. Sure, it may involve some re-working of the body mounts, bushes, and other parts...but 40 Series bodies atop 80 Series frames are fairly common, abroad. So I see no reason why the same can't be done in Pakistan.
Another option is to cut and weld the necessary parts of the 70 Series frame to that of the 40. This is more complex than a simple body swap...and has the potential for much to go wrong (in terms of keeping the chassis straight and true), so it isn't really advisable. Also, I'm not really sure about the legality of cut/weld vehicles, as far as the local police etc are concerned. However, this too, can certainly be done.
Good question you posed. I'm sure some of the other experts and adventurists can give us some sort of further input, as well.