The 55-57 manifolds are not the same. The 55 and 56 265 manifolds were total different than the 57 283 manifolds. The 265 manifolds were like a log on the side of the engine with the exit at the front. They are highly restrictive and I wouldn't use them unless it was a last resort. In 57 they changed to the "rams horn" type. They dumped straightdown in the center. The left one had a flange made to take a donut type gasket. The right one was totally flat and used a flat gasket between the manifold and the heat riser. The right manifold remained the same until about 69 when the design changed completely. The left manifold changed in about 65 . It was still a rams horn but the outlet angled toward the rear. There were some trucks that had rams horns that both sides angled toward the rear. The best one were made for Corvettes. They had 2 1/2 inch outlets and dumped straight down. You can still buy them as they are being reproduced.