I don't think it is your exhaust at all. I just meant that some day you will be breathing so well that you will eventually need exhaust improvements. I don't think that will happen until you add nitrous or a blower.
I'm actually suffering from the same problem of having runners that are too long. I have a GT-40 intake on a 351. I hope to get the Trick Flow intake at some point.
For you, either the Trick Flow Track Heat or the R-series should be good. The problem is that they will run you $450 to $500.
The less expensive alternative would be getting a new cam that goes better with your intake setup.
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351W 89 Mustang GT Convertible