No, the A9L is not the only option although it's probably the most common. No, you would not need to rewire the whole car for Mass air. There are a few pins at the ECU that need changed as well as the Mass air connector that needs added. You can get the Mass air conversion harness from LMR, it comes with the instructions on how to change your speed density car to Mass air.
Looking to put a mass air meter on your 1986-88 Mustang 5.0L? We have you covered with this mass air wiring harness!
lmr.com
As far as the Tweecer goes, I can't answer anything about it but I'm sure someone can.