I've headed off thinking about installing Shelby quarter windows for all the known reasons; can't go back, hard to install, leaks? What's the real story? If you have them, would you do it again? Thanks!
I installed a set in my '65 fastback in the early 70's - no regrets. This was my "daily driver" - my other two cars were '66 GT350's. I couldn't get used to driving the '65 without the qtr windows - so I put a set in (along with a fresh 289 HiPo, built C4, 1" sway bar, and Shelby Steering arms).
I can tell you the quality of the original installations wasn't all that great. I couldn't believe the hack job you see when you take off the internal panels, wow. I even found a chisel behind the lower interior quarters, right under the air duct for the rear brakes. The high-quality Shelby American installation also left mine with a minor leak.
Since my car came with them, I can't testify as to how hard it would be to do. I'm not sure I'd do it unless I was creating an actual clone. I like the fastback louvered vent stockers and would keep them. Like you said, you can't go back.
They are not a piece of cake but not that bad either, they fit like Sh-- but so did the originals. They may fit a bit better in fact. Would I do it well I did. As too going back, anything you do you can go back if you want to bad enough. does improve rear vison some.