There are numbers stamped into the harmonic balancer, if you can't see them, take some sand paper and clean the balancer off, you should see the pointer that points at what the timing is when you look down from the top of the engine bay. The best way I've found to see your mark, is to use a white or yellow grease marker and put a line on the balancer where the particular degrees you want to set it on.
I believe it is 10 degrees BTDC
Click the link and then choose your model Mustang.
220 horsepower
The timing mark is not at the arrow, but farther up towards the top of the motor ( a straight bar on the same piece as the arrow) The timing is set with the spout connector out ( gray plug near the distributor) Hope this clears up your question
According to fordparts.com the 1989 Ford Mustang GT has ( 1 ) oxygen sensor
stock base timing is 8 degrees before TDC
10 degrees before top dead center(BTDC) with the SPOUT connector unplugged.
It is the same as the 351W, 1-3-7-2-6-5-4-8
If it is the 5.0 L it is the HO
Cops - 1989 Ho Ho Ho 25-2 was released on: USA: 22 December 2012
Cops - 1989 Ho Ho Ho 9 24-11 was released on: USA: 10 December 2011
220 horsepower