"Sounds like you have a 25"
"Sounds like you have a 25" shaft (X-long) and want to convert it to a 20" shaft (long). We have done this conversion a bunch of times. You need a shorter drive shaft ($350 list), a shorter shift shaft ($130 list), and a shorter water pickup tube ($45 list). Time to perform the conversion is 2 - 4 hours for a reasonably competent mechanic with a decent set of tools.
If you haven't torn apart a lower unit before, you may want to leave it to a shop, or buy a manual and go for it. It's not too hard, it's just not for the faint hearted.
To replace the water pickup tube, you need to remove the lower unit, and then the mid-section cowling. Once the mid section cowling is off, you will see that the water pickup tube is held down by a tang sticking out of the exhaust pipe. You need to remove the exhaust pipe to drop the pickup tube (just 3 screws). Again, it's not too hard, but you should purchase a shop manual if you plan to do it yourself.
With the water pick-up tube, drive shaft and shift shaft replaced, simply reassemble lower unit onto engine minus the 5" spacer.
Once you've done it once, the next time will be easy!!"