340 HP's vs headers

What the power difference is between 340 manifolds and headers will vary dramatically depending on the engine. If its a stroker with big cam and fully ported heads you will be giving up a ton of power with the manifolds. If its a mild 318 probably would not be very noticeable.

The Doug and TTI header collectors line right up with the two humps in the trans cross members which is probably where your existing exhaust is routed. The cheap headers do not line up.

And like waggin says, you will probably want to run a mini starter with headers. If you have a pick and pull junk yard in your area you can get one off a Dakota for cheap. But yeah headers can be a pain to get installed and they make working on the car difficult at times.

If you want max performance or think you will do an engine upgrade later go with headers. If you want easy maintenance and are willing to give up a little power go with 340 manifolds. If your 318 is mild I don't think you will be losing any noticeable power with the manifolds.