'65 through '68 Galaxies are considered third generation, but the '68 model had slightly different sheet metal. The door panels may have slight differences between years, but '65-'67 model year door panels will interchange with one another. XL package doesn't mean anything as far as the size of the door panels. Be aware that you'll need the metal top pieces from the original door panels, and have to attach the new door panel to the old metal pieces. Here's a thread that tells how to do it: CLICK HERE
I have installed Dearborn Classic products before, one time I had good luck with them, and another time I had to trim excess fabric out of a seat cover to make it fit. I'm sure you won't have any problems with door panels. Mac's Antique Auto Parts is another supplier.