I always felt that the so called 4 seater (seats in the back) was put in to lower the insurance. ei premiums are high for sports cars that seat two as compare to sports cars that seat 4 (even though the 3rd and 4th seats are not very ergonomic.

p911

From what I remember, the 1958 TBird was not all that successful. In fact, if Ford had continued the two seater, GM would have likely discontinued the Corvette because from 1955 - 1957 the TBird was outselling Corvette on the order of 5 - 10/1. With that change in 1958 Ford gave new life to the Corvette and essentially turned over the domestic sports car market to GM. I have owned a couple of Corvettes (and still drive one) but the 1955 TBird was one fine-looking car (without the continental kit).