Huh? Who was talking about heads? Certainly not needed to get a Fox into the 13's. Not to offend, it sounds like you are relatively new to the Mustang scene.
There have been countless people, including myself that back in the day bought their fox body mustangs new, and with Bob Cosby's recipe went deep into the 13's for well under a grand. I bought a brand new 89 with 800 miles on it and went from 14.1x's stock to 13.1x's over 3 months at Moroso. Guys are still doing it to this day. I think Sharad broke into the 12's withouth removing the valve covers.
Bob's check list for 12's/low 13's
-26x8 Slicks
- 3.73 or 4.10 Gear
- Short accessory belt
- Off-road exhaust
- Headers (prefer long-tubes)
- Underdrive pulleys
- Remove the air silencer
- Ram-air/some sort of cold-air inflow, minimum K&N
- If a Mass-air car, a larger sensor
- 65mm TB with matching EGR plate
- Remove front swaybar
- Remove the clutch fan at the track
- Advance timing to approx 13 deg
- Good tune-up (new plugs, wires, rotor)
- Remove all loose items in car (spare tire, jack, etc)
-Courage. If you're scared of breaking it, you probably won't run the number!