GMC water pumps are an old ball. Seem to be in short supply and expensive. Mine was junk. Sent it off to be rebuilt. I did not remove the back cover when it returned. It looks like the first picture.
You do not mention your chassis size ? 100 to 350 series GMC in 1947-1950 shop manual says the stock cooling system was a non pressure ( atmospheric) system. There a note : All except the 100 series can be changed to pressure system. A larger core is used on the pressure system. If it is pressure system it has a 4 psi cap And has a built in recovery tank.
My 100 is a non pressure system. It’s impossible to see coolant flow in the stock radiator. The top hose is several inches from the filler neck. I run a 180* tstat.
Shop manual is very confusing on the water pump. See the attached pictures. From the pictures there appears to be two different pumps. There is no mention of two different pumps in the text. The pictures show the impeller in two different orientations.
Your picture looks suspect as far as the impeller orientation. The leaking at the weep hole is an obvious concern.
On the blown head gasket. Was the head and or block surfaced on the last repair ? Since the original head gasket was blown...........the head or block might not be flat in the blown area.
The block: The lower back water passages in the block are well known to blocked with rust and debris
