You don't really have a spare transmission, wording is tricky. The '78 R100 bike has a long input shaft transmission. The '83 R80 bike has a short input shaft transmission. The two transmissions go with their respective flywheel/clutch set ups. You can change the transmissions if you also change the flywheel/clutch. So sounds like you get what you want out of this. The lighter clutch carrier and a working transmission.
The lighter clutch swap is a performance improvement many desire. I'm not sure but you may notice some more engine vibration.