I seem to remember that The Camaro Research Group has a good write-up on seat-belts/seat-belt options in the 1967 Camaro.
I think you might be able to check the RPO distribution to get an idea for rarity. Otherwise, I don't think you'll get a definitive answer as option combo data aren't available. That is, you can see how many SS cars, how many RS cars, and how many shoulder belt cars, but you cannot see how many cars had the combination of these options.
Anyway, the CRG should provide you with some of the information you're after.
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