If you think the main shaft is around 6" long, you're not looking at the main shaft; you're looking at the input shaft.
As Racer_X said earlier, there is no difference in length between the 0.82 and the 0.64 main shafts. You can therefore complete the ratio swap without changing out the input shaft, bellhousing or driveshaft.
If you then choose to change bellhousing etc, that's up to you, but if your friend has the MAIN shaft and gear you might be making things more complex than they need to be.
Here's a main shaft and gear for sale in the classifieds section for an image of the parts.
PhotoPost Classifieds - .82 main shaft and 5 th gear - Powered by PhotoPost Classifieds