If you haven't installed the freeze plugs yet, check the coolant passages for rust build-up.
Rust tends to settle at the rear of the engine which may inhibit coolant flow.
A hot area in the block can cause the head to overheat and warp.
Also remove the drain petcock(s) and water jacket plugs to check for build-up.
I had a serious build-up in my block. No flow to drain the coolant.
Removed the petcock and drain plugs. Still no flow.
It took me a day of gently poking also sorts of things to break up the sludge and rust.
Hot water, compressed air to get the most of it and finally a block cleaner.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."