Right! Putting headers on one means you need to make many changes in order to take FULL advantage of headers. This is why people argue headers add "only" 10hp, which we all know is bullshit, because if you RETUNE the engine to take full advantage of headers, you will have more then 10HP difference. How much depends on the rest of the engine. But headers can affect everything from carburetor tuning to distributor tuning and lots of other things. It's all about the complete combo and adding headers CHANGES that combo, so be prepared to make other changed to take advantage of them.
