What is the difference between a production engine and a Mopar Performance crate hemi?

The reason I ask is LQK has tons of low milage hemi engines for 1/3 the cost of the crate engine. If there is something different could it be changed?

Thanks for your help in advance.
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55 Pick up 56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bee, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d