Just any old 124 driveshaft would not work. It would have to be from a manual transmission car. Your best bet would be to have one custom built.

You cannot put a pilot bearing/bushing in the flywheel. The hole in the center is much too large. If the automatic 103 crankshaft is similar to the diesels I've seen, it does not have a pilot bearing. You will have to custom machine this item. Don't even dream about putting all this work and money together without a pilot bearing. If you do, you will be measuring the transmission life in minutes.

Again, the ONLY way that I would do this transplant is with the entire donor car close at hand. If you try it any other way, this will be a long involved project and will require having another car to drive while it is being done.

Good luck,