For a long time I thought my '76 only had a single, then I discovered the lower horn and got it cleaned up a bit, re-did the connection, and lo and behold it worked! Definitely a better (and louder) sound than before. You could probably get the other horn from a salvage yard. Or e-bay.
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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.