Your timing is fine. You could bump it up to maybe 12 degrees on 89.
Check your plug wires, make sure they are not laying on the headers or near anything that could be rubbing and causing it to short out. While your at it, check other things like cap, rotor plugs. If its just starting to get a miss at certain rpms its only going to get worse as you keep driving it.
~1971 Mustang Mach 1~
~1988 Mustang GT~
~1990 Mustang GT~
~2000 Mustang GT~