I looked into this a couple of years ago and from what I've read, the kits from One Piece Products has been plagued with numerous problems, including non-delivery and even when they have been delivered, non-fitment and weakening of the door structure. Mustangs to Fear is supposed to be coming out with a kit that will address these issues.
A couple of years ago there was quite a bit of talk on this forum about those windows. As I recall there were problems with the installation. On the '65/'66 and maybe other years there are 2 metal tabs that connect the outer skin to the inner shell that must be removed which weakens the door.
I haven't heard anything about it lately. I guess due to the high cost and the installation problems the only people who use the kit are building highly modified show cars that are never driven.