RE: pipe resistance
Mike,
Did you use both o-rings. The header needs to be sandwiched between the two rings. I have never had a issue with those flanges if the oring's are installed properly and it should be tight enough to grip a stainless header. The aluminum headers they were originally designed for were thicker. You can take material off the mating surface of the outer flange and it would tighten it up even more. I always have to do this to the brand new no-leak design flanges to get them tight enough, also.
No doubt a new style no leak prevents water from entering the system if you burn up your o-rings but the regular water cooled have been used successfully for years. Some still prefer them to the new style.
If the o-rings aren't leaking the water cooled style like that engine has on it work very well.
Thank you,
Daniel