With a 26" tall tire, a 3.73 would be about as steep as I would recommend on the street.
90lx - I would take some time to think about what you want to do with this car. Write your question to include asking for the cam and gear suggestions, since both are so related to the overall setup. At the same time, be prepared to consider changing the stock stall converter if you want to go up on the cam much over stock. Some cams will say that they are good with a stock converter, but then your performance gains will be minimal at best.
I have seen plenty of late 80's - early 90's Mustangs run very well with nothing more than a gear change, and 3.73 with a 26" tall tire will wake it up, even in stock form.
Once you know what your starting with and can define your goal, you will get some good advice, probably even with some cam makes and part numbers. And remember, write a title that will grab the Ford guys attention. Good luck, Ed