Scott Smith has mentioned a couple times that you can fill the cooling system with EvapoRust. Run the engine up to operating temp and shut if off. Let it sit over night and then flush.

My only concern is getting all of the EvapoRust back out of the system. The block drain plugs would definitely need to be removed. I know on the SB engines, they are right behind the engine mounts and a PITA to get to.