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I don't think there is much difference other than iron head big blocks with AC weigh the most and aluminum head small blocks weigh the least. They're all around 3500lb+.
The above website lists most of the weights and also tells you what year was the slowest in the C3.
1975:
This was one of the slowest Corvettes ever made. Even with the L-82 option it took almost 8 seconds to get to 60mph. Quarter mile came in at 16.1 seconds and top speed was 129mph :U