The AMC AMX 390 Go Package has moved from overlooked oddball to serious contender in the classic muscle market, and recent sales show collectors are finally pricing in its performance and rarity.

This AMC muscle car only appeared in small numbers, but today it has a bigger story to tell.

Just as there was once an age of giant reptiles some 100-odd million years ago, there was once an age of American automotive giants. Although they may be dinosaurs today, the “Big Three”—General ...