On my 5.3 conversion I was having an issue with no heat inside vehicle. At first I though water pump was bad because the coolant was not circulating around the radiator. All the lines were cool after running for several minutes. I started reading on line and discovered it is common for air to get trapped in cooling system and prevent normal flow. There are several good youtubes on "burping the system".
Some symptoms are; empty system only accepting 1 gallon of coolant, no interior heat, all lines and hoses are are cool to touch and you cannot see any coolant flowing into the radiator when you look through the cap hole.
I resolved the issue by filling to system from the upper upper radiator hose. Disconnect where the hose meets the radiator inlet. By doing this I was able to add an additional gallon of coolant! You can actually hear air bubbling up as you are filling. Remember, when you start the vehicle after performing this, make sure the heater is on and set to full hot. You may be able to add even more after all the air is purged.
Easy fix. Hope this helps before you go pulling water pumps and changing thermostats....