The 914 uses unibody construction. As such, it is not nearly as easy to replace the floorpans (and other structural parts) as it is in, for example, a Bug. You wind up cutting out the bad metal and welding in new.
Making sure everything lines up correctly when you're done is important, and not completely trivial. Lots of jigs and measurement fixtures are a good idea.
Some links to help with the dimensions:
Pelican's chassis dimensions page
914 Club dimensions pages
914 Club underbody dimensions page
Good luck!!
--DD