The closed chamber Edelbrock heads are supposed to be 63 cc but actually measure out to 64-65 cc. So it does not surprise me the Eddie Magnum heads have a bigger chamber then they are suppose to.
I read on another site a person had some EQ heads Stage 3 ported by Shady Dell and they flowed 260 cfm. My ported Edelbrocks from Shady Dell flow 280 cfm. I would think ported EQ heads could get you up to 500 hp on a 4" stroke engine. For someone on a budget I think the EQ heads are a good option.
As far as Hughes goes I don't trust them. They know a lot about mopars and sell some good stuff but I would not believe everything they tell you.