When you're in there, you may as well do both. I suggest the Ford 3.73s. You will need some access to a shop press and the necessary tools for removing and installing bearings. A local machine shop should be able to do that stuff for you. A dial indicator for measuring backlash and pinion depth is also good for confirmation of the install. If you stick with Ford gears it is a good bet that all the shims will already be correct. At the very least, you should do a pattern test.
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351W 89 Mustang GT Convertible