an 85 gt has a lot more power than a 65 mustang with a stock v-8. Iwould say it a waste of money to get a i-6 if you want a v-8. I remember seeing a 65 with the i-6 17 or 18 second quarter mile, which is about what my dads camry runs. I would fing a mustang with a stock v-8 and then add more power when i felt it was nesasary. and thats exactly what i did. i got mine when i was 15