The exploded view that you posted is for a Quiet Line tractor and has been modified from the original version by the owner of the site that this illustration came from.
The correct length drive shaft for a 108 with a creeper is 14 1/2".
According to my research, some Wide Frame tractors (86, 108 and 128) came with the spacer and some did not (I'm guessing that somewhere during production there was a change, but I can find no information on when this change was made).
In any event, spacer/s or not, this does not affect the over-all length of the shaft, it does however change the location of the third hole in the shaft.
It is my opinion that you should, either send your old shaft to whoever is making the new one so that it can be duplicated (best option), or
accurately measure the
exact distance from center to center of the 2nd and 3rd holes and send this information to the maker.
Here is the link for the correct drive shaft/clutch exploded view for your machine (which is the exact same illustration that Cub Cadet uses on their parts site).
http://www.partstree.com/parts/?lc=c...&dn=0012400012