If you don't mind my asking, how did you block off the port so you didn't fill the whole thing with aluminum? Did you use some sort of putty, clay or plaster?
edit: Oh, never mind. Now I get it. You poured from the exhaust side. Sorry.
Exactly. You just watch as you pour and stop when you need to. If you over pour a little you just take a die grinder and grind it down to match the other exhaust ports.