i dont like using anti-sieze because it collects dirt like no other and if you were ever to touch it then it rubs off. you might have to get a high pressure washer or get a shop to heat soak (cant think of the actual term for it) it. my engine block was 1000x worse than that and my buddy cleaned it up at his shop with that heat soak thing
The heat soak thing HazMat404 is talking about is a hot tank. You can get that intake hot tanked for pretty cheap and it will be about as clean as it's going to get. Just ask your local machine shop, they usually will have a hot tank.