Well I can tell you this that a backfire is caused by an over rich condition, if it were backfiring threw the intake then you would have a lean condition. I would check your plugs to see where you are with your fuel issues to "VERIFY" first. Then you know where you need to start. You might also want to check your timing as well, I hope this helps you a bit, but like I said before VERIFY what the problem is first then go from there.
Well here is a place that I found to be pretty good, no cores and a great price. Just follow the link! http://www.adccylinderheads.com/sit...roduct/CH-408-B Now that is for the 2.8 but Im sure they have more. Good luck