Think about it like this; The radiator which does all the cooling has like 1-3/4" hoses, right? The rear heater passage is like 1/2" I.D. Even if you put a custom fitting in the head you might could get a 3/4" or 7/8" hose on it, I just don't think it will ever flow enough to make a difference. I would have to assume the engineers designed a specific "flow path" through the block and heads. Changing that flow path as far as I can tell does not improve cooling.

I will say that it seems like everyones cooling problems are different and are not always solved by a common solution. I had a cooling problem that I tried "everything" to solve. Ended up that a simple flex fan did the trick. I was getting inadequate air flow through the radiator at highway speeds. The clutch fan might have even been restricting air flow at speed. I would get in town it would cool back down some. Put on the flex fan and had to take out the 160° t-stat and put back the 180° t-stat cuz it would not get over 160°. Now it stays a 180°.