It's part of a project. Sometimes it turns out this way. Compression would not be right if the headgasket was blown and you would be blowing out coolant. Just like you said you are using a pretty beat up radiator. That's the problem. Make sure you are getting plenty of air with whatever fan setup your are using, do you have a fan shroud? Buy a new radiator since you are using a used one - they become inefficient when they are old, clogg, and many other things. I think that is your only problem. Is the waterpump new? Try changing the thermostat. That's all you pretty much have to make sure of. I would say a new radiator and making sure the fan / fans are working good = problem solved.