Pontiac never built a 428-cubic-inch Firebird, which makes this 1967 convertible’s engine bay a genuine puzzle until you realize the builder stayed entirely within Pontiac’s own performance parts family. Backed by a four-speed manual and topped with Edelbrock aluminum heads, this is a big-inch build with a cleaner pedigree than most cross-brand engine swaps. Read on to find out where that 428 actually came from.