In our thread on the mounting of the towing attachment it was stated that it isn't the spring that is longer, but that the mounting brackets reach further inboard on the F/C15 as compared to the F/C15A and that the same spring assembly is used for both types. (see posting dated March 10 2007 for a photo) Finding the 2 wheel drive brackets could be enough of a chore to make a new/modified spring very atractive.
See:
http://www.mapleleafup.org/forums/sh...&threadid=7096