I have been researching this as part of another cooling thread and have a 717 engine with the top taken off to take some pictures that I put all kinds of pretty arrows/labels on, but do not seem to be able to get them uploaded/posted yet. I can advise here when I get them uploaded if desired.
The cooling flow is into the IN at the rear top of the head. The head/cover is kinda split for water flow up the middle with a port side that outlets to the port directly opposite it on the front of the head to feed the tuned exhaust. Inside the top of the head cover this port side flow feeds internal ports that go down into the cylinder cooling jacket. The cylinder cooling jacket is not baffled, but has one set of 4 outlets at the starboard side going back into the top cover and out the OUT connector at the rear of the top of the head. The other outlet from the cylinder cooling jacket is under the exhaust ports, thru the bottom of the exhaust header to the 90 degree connector to which the tubing to the tee, outlet and pee hole is connected. I hope this makes more sense than confuses.
As Brock said above or a blockage in the 90 degree connector or exhaust header.
The 90 degree fittings on the exhaust header and tuned exhaust are specific sizes internally to properly regulate the water flow to the various parts and thus should not be replaced with standard pipe fittings.
Hope this helps a little.
Rod