If you're looking to just repair the floor and move on with your project, you might get some repop pieces from the earlier models.
I got lucky and found some pieces by Mr. Mustang, which CJPP had listed as '71-'73 replacement pieces. They fit perfectly, of course, but had some slightly different shapes to them. I've recently noticed that they are no longer available for '71-'73, but the same pieces I purchased years ago are listed for earlier model years.
http://www.cjponyparts.com/mr-mustang-floor-extension-rear-driver-side-1966-1970/p/M151L/
http://www.cjponyparts.com/mr-mustang-floor-extension-rear-passenger-side-1966-1970/p/M151R/
What I don't get is that if Dynacorn is stamping out full floor pans, why can't they have a smaller set-up to stamp out some new rear seat platforms?
Here's how mine turned out (before finishing the welding, sealing, and adding RaaMat). The passenger side rear seat platform needed complete replacement, so I used the panel I'd bought from CJPP (the same ones listed above... seriously). The driver side only needed some repair around the torque box area, so I just got a piece of sheet metal and shaped it to fit.
If you don't need to replace either of the pieces entirely, I would suggest cutting out the bad parts, fabricate some replacement 'patches,' and weld them in in the same manner you're replacing the pans. You're taking your time and doing it right, so it's not like the next guy will come along and wonder WTF you did.
I hope this helps.