My father in law is slowly finishing the rebuild of his 6.5
He will replace his 506 with a 141. He is quite happy about that, as the 141 are known to be better than the 506 (no oil squirt).

That being said, he wants to do the cooling mod on the heads.

Usually, to do so, we have to open the rear ports on each heads and then route a hose to the T-stats housing or to the waterpump.

He says routing to the T-stats housing is not worth the pain as it will connect to hot water circuit.

Then, he says he will interconnect both heads so that will improve flow between the 2 heads.

I pretend that will not improve anything.

What's your opinion guys ?

I would think going to the water pump outlet somewhere would be the best since it pumps in cool water from the radiator. Dont know if you can tap into the pump outlet or not tho. However heath diesel just published some information stating that a single thermostat housing with a HO water pump improved flow in the heads dramatically verses all the other setups, including the HO pump with the dual thermostats. I guess the dual thermostats allows the water to flow through the heads too quickly to remove enough heat and the single t stat slows it down a little and allows the heads to fill up more completely and remove more heat. Makes sense to me.

This is the direction i'm going to go.

Unless he is gonna pull heavy,IMO, the whole head bypass shebang aint worth the hassle.
just the HO pump and 9 blade fan,a cooler T-stat and a clean rad should be all one needs. A 3.5" D-pipe and a TM will drop the EGT's on the other end.