Tom, I don't think the temps are the same because you are moving water away from the #6 exhaust valve area. Without the flow back to the thermostat housing from that area I believe that spot of the head runs hotter because of the lack of good flow away from it. It may never have been a problem with a 228 or 248 in a pickup but a duece and a half loaded under a full load could have been a problem and GMC engineers fixed it with the tube.
Another point is when many of the guys mill a head, especialy over .100" they lose the small water directors/aimers which aim the water at the exhaust valves. I have always made sure mine stayed in place. Anyway since we added the line I've never had a heating problem at the drags with my coupe or running the roadster to the 3 mile with 9 gallon water tank starting at 120/130 degrees.
If I ever figure out how to put a picture in here I'll include one of that side of the engine...JD
