There are headers and then there are headers. Alas many people (often myself included) purchase based more on price than anything else, and in the case of headers there are a lot of cheap no-name ebay knockoffs out there.
It can be very true that headers make changing plugs difficult, I've had that problem on both my Corvette and my elco. I found for example that getting the plugs out of the vette was far easier to do from below than from above, much the same with the elco at least on a few of them.
It is also true that for a mild motor, old iron manifolds are more than fine, they are quiet and durable beyond compare.
However the moment you start wanting to improve on performance there are few iron manifolds that can be recommended, some may have larger passages but log manifolds regardless of passage size represent a restriction. The old rams horns manifolds found on Corvettes and early chevies are probably among the best iron manifolds you can use, unfortunately the rams horn wont work on an Elco. Old chrysler hemis had great manifolds too IIRC
That leaves us with headers.
Like mentioned above alot of headers now come with thin tubing usually 16-18 ga, and they also usually have really flimsy flanges around 3/16" - For those of you unfamiliar with metal gauge, the bigger the number the thinner the wall my AISC manual shows.
US std ga for uncoated materials.
14ga = .0747" (.0785 zinc coated)
16ga = .0598" (.0635 zinc coated)
18ga = .0478" (.0516 zinc coated)
You can see that 18 ga. is significantly thinner than 14 ga. A quick study of the Jegs catalog on my desk here at work shows the following.
- Hedman painted street/strip - 18 ga 3/16" flange (yuk this is super flimsy)
- Hedman elite = 14 ga. 3/8" flange (this is a good set of dimensions)
- Hooker competition = 16 ga. 5/16" (falls between the two)
- Jegs coated - 18 ga.
- Jegs painted - 16 ga. 5/16" (I wonder why there is a difference)
- Dougs 16 ga. 3/8" flanges (quality header)
- Patriot 16 ga. 3/8" flanges
Some of the mfr's dont list their material flowtech for example. A thin walled header like 18 ga. will sound tinny and you will be able to hear the exhaust in each tube, under the hood they sound bad to me, and of course they can wear out much quicker than a thicker header.
My minimum standards would be nothing less than a 5/16" (3/8" being better) flange and nothing less than 16 ga. (14 ga. would be better.) These would be my recommendations if you were looking for a header.
Different styles of headers may offer better plug access (shorty, block hugger etc..), but for the most part I always assume people are talking full length headers when they discuss plug access woes. Another issue with headers is fitment, I have it on good authority that for example one header manufacturer actually uses the pattern for a full size b-body car to make the 1-5/8" primary headers for an A-body. They determined that the fit was "good enough" which really means constant dimpling to miss stupid things like the steering box which should be easy to miss.
finally a jet-hot or other ceramic coating will increase the apparant tube thickness - they run quieter and cooler than uncoated ones.