Trefty is 100% correct. If they are Torque to Yield, they are only meant to be used once and the stretch they experience from the initial final-stage torquing makes them unreliable to use again.
ARP head bolts are not T-T-Y, and can be reused. Try ARP.
I agree with the above. DON"T reuse the stock head bolts unless you like taking big chances. The factory only meant them to be used once.
So are they torque to yield (Ford) bolts or ARPs
i reused some torque to yield head bolts on a 20.00 rally car i bought with a blown head gasket and it hasnt caused a problem...but since i assume your mustang was more than 20.00, you should replace em