Steve,
Just things to check on:
Look on the under side of the rim, where the bezel wraps around under the rim, and is protected by the rubber donut. If the rim is black there, then it's just a case of too much sun. My memory says the black instrument bezels were particularly sensitive to sunlight and weather, losing their flat black finish at a pretty rapid rate. Another factor to consider is that the parts book for the 350 lists only 1 part number for the speedo and for the tach, no separate numbers for the GTO.