320HP Magnum Crate w/ X-heads

Brooks James said

Im confused, the rating for the Summit engine is 320 hp.
I have seen several dyno pulls with a stock 360 mag short block with headers, holley 750 a dual plane stock cam, come in at 300 hp Summit advertises 320 hp with same set up, where is the extra 20 hp coming from ???

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That's my question too. The bore on Summit's advert is 4.060". Says the cam is stock Magnum grind. stock heads.

I talked with my Dad a little more about the cam. He seems to think he will be happy with the stock grind. He wants to drive long distances and get good fuel mileage. He's older so his go fast days are done.
I told him those cam specs are at the bottom end of the spectrum and that is with the 9:1 compression and closed chambered heads. He's GOT TO HAVE the x-heads, for some reason, and the main centerpiece of his vision is the LD6B six pack. Okay, I says, fair enough but the stock magnum piston and it's height isn't going to work with the open chamber X-head. I consider the change to the KB dome piston is almost a have to. They are $400 and I would like to get at least that much out of the fresh Magnum heads.
I am thinking we can get away with a piston change (leave the little cam), pushrods, and that Mancini LA/Magnum rocker gear.