273 / 318 Exhaust Manifold Question

Swinger 340 said

The 340 (68' - 71') output to tail pipe is larger, flat and no cone, the sweeps are similar. A guess is the openings at the head may also be larger on the early 340's. If I were to use the best cast iron exhaust instead of headers, I would buy the after market ones for the early 340, around $600.00. Don't forget the special bolts and cone washers to go with them.

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The problem with using the '68-up 340 exhaust manifolds on a '66 and older A-body 273/318 is that they don't fit the body very well, you need to custom make the whole exhaust system, and you'll have a hard time keeping head to manifold gaskets from blowing out due to the very thin sealing area on the mismatched port sizes between what a 340 head has for a port, and what the 273/318 head has.