Kevin,
Sorry for the delay in answering but I have been in hospital with kidney stones for the last two weeks, two operations, 3 stones , one over 3/8 inch in diameter. #$%^& ing painful is all I can say.
Regarding the thermostst bypass, on most aftermarket inlet manifolds, the rear of the heads are not connected as they are with the standard Ford manifolds. This does not allow steam to escape and this area increases in temperature and is sometimes said to "boil". Very common practice in top level racing is to use a by-pass to allow the steam to escape and keep the head temperature constant over its length. We have done this and the AN fitting you have found is it point where the water by-passes reenter the syatem.
Any other problems please contact me directly.
Best wishes,
Robert