I was being a bit facetious about them taking the car. My point was that failure isn't a big deal so don't invest a lot of money trying to pass. Wait until you fail to do anything, and if you don't fail, you haven't wasted anything.
Yes, running lean generates more heat. This is why you can get more power running on the rich side. Since the combustion chamber temp is kept down, you can actually squeeze more air into the cylinder. Water injection does the same thing but without generating the extra HC in the exhaust.
Your best bet is to keep with the stock settings of the computer and don't try to tweak anything. The other tip about making sure your cats are hot before the test is a good one. Since it appears that you don't have an air pump, that could mean trouble. The cats need oxygen to catylize. Without it, they won't work as efficiently and could lead to failing the test. Get one from someplace like
www.stangparts.com or even a local wrecker.
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351W 89 Mustang GT Convertible