Hello Harry,
I have a '66 GT with the same situation on the windows, except that I just
recently replaced the plastic street side window with Corning. The plastic
bowed at the bottom and you will find that after awhile it will bow at the
corners too. There are only two Phillips fasteners on the street side window
because with the Corning glass that was all that was needed. However, I have
glass on the rear window, where there are three fasteners, but for a long
time I was missing a retaining clip, so could only use two fasteners. I
never had leakage or any other problem just using two. You might consider
removing the middle Phillips from the rear window and moving it over to the
street side until you find a new one.
Forrest