Headers VS 340 manifolds
74, let me tell you some facts and you can decide. When I bought my 69 340 four speed Swinger in 77, it had headers on it. Dad had an automatic 69. His would whoop mine at the track. Both ran about 14.30s or so with good tires. Both were dead STOCK. Mine had headers on it. I cut one side off and went back to the strip with one manifold and one header. It was about a tenth quicker than it had been. So, I cut the other side off and put on the other stock manifold and it picked up almost another tenth; the headers have never been back on my car.
For a 318 that you want to run and sound great, put the stock 340 manifolds on with 2.5" front pipes if you want a bit more flow and never look back. Regardless of what headers cost, these will work almost the same (if not better), last forever, and never give you trouble and never need resealed.
I hate headers on STOCK engines. The gain they provide is not worth the hassle they create on STOCK engines. Modified, different story.
Tom