1973; 318 vs 340 manifolds

It depends on what kind of power you want to make. On a stock 340, I would say the 318 manifolds will not hurt horsepower that much. And yes, factory 340 manifolds are very expensive. $400-$700 range. Can't beat the performance of headers, especially if you have done any kind of performance work on the 340. tti's are probably the best and very pricey. Lots of different header manufacturers though. There are a few threads around here on what headers fit best and why.