this pair of 1969 Boss mustangs went for half a million dollars at Barrett-Jackson. the car on the left is the 302 car, a lighter weight, racing-heritage boss. it's hybrid v8 was part cleveland/part windsor, with big port deep breathing heads atop a 4-bolt windsor 302 block. raced in scca racing for a few years with lots of wins, it's probably the most balanced of all the factory ford mustang race cars of old.
the 429 car was the car assembled by Car Kraft for ford, using the enormous Boss 429 engine. recognizable by any car freak, the engine features the semi-hemi heads. the 429 itself isn't very big, actually smaller physically than most DOHC engines today, but adding the wild Boss heads to it required Car Kraft to actually reposition the shock towers for it to fit. ultimately a better racing engine than a street engine, ford only needed to make so many of them to qualify the engine for racing, only to have it outlawed. ford lost money on every one of them.