According to Gordon Maltby (Porsche 356 and RS Spyders, copyright 1991)
Most of the differences between a 1957 and 1958 were mechanical (as outlined earlier in this thread, especially by Mike and Glenn) but there were some body/trim changes:
1957: The introduction of the T-2 body. Teardrop tail lights were introduced in place of the bee-hives, the rear license light was a shine-up in place of a shine-down. Some misc. trim pieces were phased in during the year as stock was depleted (hard to say what and when). Slightly higher top bows and slightly larger rear window. Glaspar (from California) hard top introduced at dealers in America. 644 transmission introduced - the so-called "tunnel case" because the innards were installed through one open end of the housing, which was one-piece, rather than a split case.
1958: Convertible "D" (for Drauz, who produced the body) introduced, becoming the "darling" of Zuffenhausen, who never liked the "cheaper" Speedster. The "D" was only produced for two years, included a higher windshield and roll-up windows and a significantly plusher interior. For the 1958 Speedster, carb linkage was from lever arms operating from a shaft in front of the fan shroud. Shifter was moved back, while heater knob was moved slightly forward. Slightly larger 425mm steering wheel installed (most of these were changed to Nardi's in the states) along with a ZF steering box. Slightly different outside door handle shape (undefined by Maltby but probably similar to the 356 "C" version). Changed to the type 716 transmission (stronger, but tougher shifting) in December, 1958.
In September, 1959, they introduced the "C" model for 1960, ending the Speedster.
Put all these posts together and you'll come pretty close, I guess.