I have found that block prep is key to keeping an engine cool. Like most of the motors we have in our cars most if not all are really old and filled with all sorts of gunk. Most of us go for the simple rebuild and after all is done we suffer from traffic overheating. One thing I do with any project motor is to clean the water passages/water jackets with muric acid. Real simple to do, doesn't take much time and is inexpensive. It's a way to get to the area of the block where your standard hot tanking or motor pressure washer can't get to.