Just some random thoughts. The over heating and the oil being pulled into the intake ports tell me the manifold is not sealing as you surmised. Which might be creating a lean condition causing your overheating. Another thing. Pushing coolant out is just expansion. A good cap and a recovery system are important if you don't use or have eliminated them. If the intake isn't sealing (assuming a sbc here), when you reinstall it, use rtv silicone instead of end gaskets. But before you do that, a question. Have the heads been milled more than a cleanup? If so, you may need to have the intake done as well. (milled) When its all back together, check your vaccuum. If it isn't within acceptable limits at idle, you may have timing issues which in turn lead to over heating. Without knowing the whole package/story it is a tricky thing to offer advice on. Check back and lemme know how it works out.