Re: force 125 short shaft to long shaft?
First off, they never mad a short shaft engine in that size (except for racing) The standard is a 20 inch LONG shaft and a 25 inch EXTRA LONG shaft engine.
Your 88 uses a single one piece drive shaft. They did make extra long versions (hell, I have a 1979 Chrysler 25 inch shaft engine) BUT, you need to remove the old drive shaft and install a longer one. Way too much fuss and bother. better to find a 89 or later two piece lower unit and simply buy the longer upper drive shaft. I have never seen a 5 inch extension kit--that doesn't mean they weren't made, but I have never seen one. Thus, they would be difficult to find.
Understand though that you will need a new mid leg and front yoke assembly to convert. Oh, wait! I just re read your post: You HAVE an extra long engine and you bought a 2 piece drive shaft lower unit with a regular shaft correct?
I have a couple of long drive shafts. If this is what you need, send me a private message.