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.
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