Anyone opened these up? , water ports in head gaskets seems pretty small , or maybe just open the holes in gasket feeding the rear cylinders?

These are the 1009 4.590" bore.

I asked Scott this very same Ques and this is what he told me.

The water needs to go to the back of the block, then gets directed up, through the heads, and back to the rad. If you open those gaskets holes to match the block it will send water to the head though them and create a steam pocket at the back of the motor cuasing that area to get to hot.

Yea that makes sense , which is why i'm thinking of opening the holes toward the rear , am also thinking this would move cooler water toward back of block anyhow , i spose i could measure the holes in gasket & calculate the flow rate , just not convinced these holes are big enough.

Open them up to your own risk. Lots of $ has been spent on research to find out how large those holes need to be and to make sure the coolant goes to the places it needs to go. If you're worried about too much heat, run a reverse cooling setup so it cools the heads first.
No need to reinvent the wheel, it's already been done.

i run the reverse flow pump & found if i drilled a hole in the gasket by where the 2 exhausts are together that improved cooling tremendously. the holes up by the lifter valley were just to help clear the air bubbles.