66-67 cars are generally not grouped together. 65-66 are very similar, as are 67-68.
I've owned two 65's, one coupe and one fastback, and I've restored 67 and 68 convertibles. The 67-68's are a little larger, a little more comfortable and not nearly as good looking. This is not my opinion, it is a fact. 68 and up cars also have a collapsible steering column, which may be of interest to you, if you plan on hitting on anything with it.
If you don't care about numbers matching, then don't pay a premium for an original GT. Just build a good car up to GT (or better) specs. I would, however, start with an original V8 car. C code V8's are common. GT's were A or K, and any A or K demands more money. Even the basic disc brake conversion kits are better than the original GT discs.
The front cowl will leak, unless it has been repaired, and it is not an easy fix. You should check the rear "cowl" area (the area between the rear window and trunk opening), especially on a fastback. Take a flashlight, stick your head in the trunk and look up underneath the panel. There should be stamped reinforcements panels visible up there. A dried out, leaky rear window seal will lead to rust up there. As far as I know, replacement panels for that area are not available. There are enough donor coupes out there to find a good replacement piece, but donor fastbacks are scarce.
Just about every other piece on the car is reproduced, so they are good candidates for a first restoration project, though convertibles are harder to get right than coupes and fastbacks.
There was a time in my life when I knew more about early Mustangs than I did about anything else. I love to look at them, but even when they are well set up, I found that driving them was disappointing compared to even semi-modern cars.
Please do not build another Shelby clone.