320HP Magnum Crate w/ X-heads

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.

If I could pick only one part to change in the magnum it would be the cam. You can get a cam 2 or 3 sizes bigger and not effect mileage and would be about the equivalent of the stock 340 cam. I run this Hughes cam in my 5.9 combined with EQ heads stock valve size, everything else stock no tune and it made a HUGE difference in the top end. Engine was a total dog and would fall on its face passing or towing before, now it is fun to drive and owns the interstate. With your carbs I would get a 110 centerline cam instead of the 114.
https://www.hughesengines.com/Index...Z2UgTWFnbnVt&level1=Q2Ftc2hhZnQ=&partid=30212