I have the GT PBRs on my '68. I had them available, and they fit fine under 15" wheels. Perhaps I'll go Cobra when I get bigger wheels, but at the time I was not willing to do that.
..and, despite prevailing opinion, they are not an automatic death sentence. Some have even opentracked with them and lived to post another day
The K/H 4 piston calipers are a better option then the GT/V6 calipers. They have more stopping power and fit smaller wheels. Also, the smaller PBR calipers just plain suck.