It's all relative. An '89 GT vert running perfectly is not that fast. I'm not picking on you. I own an '89 5.0 as well.
The first thing to do is to see if it is running properly. If it's getting fuel, you swap the plugs, and then you verify that it's all working properly, you should then check the timing and set it to 14° as was mentioned.
That's really all you can do.
My '89 LX hatch with a stock 302 was pretty slow. Probably a 14.0 @ 99 car. Then I got a wild hair and started doing some tweaking. I dropped some weight and added UPR suspension, drag radials, and a MAC cold air, then I took it to the track. With the motor still stock, I went 12.91 @ 103.7. Here's an article I wrote about it.
1989 Ford Mustang LX - Fox Buildup
What's crazy about it is that the butt dyno thought the car was a beast after the tweaks... but I removed the entire drivetrain from the car, put it in a Fox coupe, added an 85mm MAF sensor, and the car made 236rwhp and 303 lb/ft of torque. 236hp!!!
So like I said, it's all relative. As was already mentioned, I thought about the timing, the rear gear, and what tires you're running. An '89 Vert like yours can run in the 13s with the timing set at 14°, a 3.73 gear (the 4.10 will be fine on drag radials), and a decent tire.