Are 1984 Mustangs more reliable than Japanese cars?
There are several concerns here. The Mustang LX of that year can come with the same V8 that the GT had, so if you have not been driving for a least three years, you should check how much insurance will cost. It could be very expensive.
Second, and in direct answer to your question, no the Ford will not be as reliable as the same year and same mileage Japanese car. It probably will not be as reliable as one with double that mileage, either.