I removed my original standard Chevy 350 water neck and replaced it with probably one of the most expensive water necks there is! Here is a link to the product I bought:
With mid engine vehicles it appears a challenge to keep air out of the system as the radiator is mounted lower than the engine. So this Aerospace Components AC-FN2 Chevy Water Filler Neck hopefully solves this issue. So everything up front is original as in the radiator/cap/overflow hose and the cap is a 25lb psi cap. You can see the before and after pictures attached below.
At the engine you will see the attached images where I removed the original filler neck, installed the new Aerospace aluminum neck with a 20lb psi cap and overflow tube. I ran the car yesterday and it never overheated. I have not installed the overflow/expansion tank yet.
Q for the more experienced. Does the overflow tank itself need to be the highest point? I was just going to mount it low and keep my filler neck at the highest point as seen in the attached images.
The overflow tank can be lower. I've had them up to a foot below the cap and never had any issues with the system drawing water back into the block on cool-down. You could probably go lower, but I never did. On my overflow tank, I brought the hose from the cap into the bottom of the tank, and then the fitting at the top of the tank was hosed to the back of the car for dumping. That way the first thing that will be pushed out of the block will be air, it will bubble up to the top and out, and when the engine draws the water back from the bottom of the tank, it is all coolant.