Iron 273 4 barrel manifolds

I use them on my street cars. There were 3 main casting numbers used in A-bodies. The '65 model was unique. In '66-7 they used 2536563 for most applications, and a 2536771 for cars with C.A.P. (Cleaner Air Package). I don't know of any performance differences between the three. I won't get into the D-Dart or marine/industrial application discussion.....