This manifold is for a 68-70 425 Engine not 455
The 1969-72 exhaust manifolds feature a divider between the center exhaust ports. The 1967 and earlier models do not have this feature. Also, all manifolds except 1970 to 1972 will have a heat riser (shutoff) on the left-side exhaust manifold.
I just found a Left side Dual Exhaust Manifold for my 394...now I must make a shut-off-plate for the Right side Exhaust Manifold, since they are the same on single & dual exhaust systems.
THORTON REPRODUCTIONS has your LEFT EXHAUST MANIFOLD.......
http://gawain.membrane.com/~thornton/manifold.html