Good answers guys, it should really have been over in the v8 forums, and I would really think that March and probably several other companys make ram air kits to fit that model GT. From my personal experience with March ram air on my v6 car though, you wouldn't notice much difference except maybe when passing, not a lot then.
A little off topic, but my first good car was a 64 1/2 Mustang Convertable, I turned 14 in November of 64. I got that car in 67 when I was in High School, Graduated in 69, I paid for her working part time in a hosiery mill after school. Started out at $1.60 per hour, big money, huh guys? Used to clear like $60 a week, had plenty of money, my payments were like $60 a month.