How To Determine a Matching Numbers Engine and Chassis

Hi All,

Sorry if the title isn't worded correctly. I'm wondering if anyone can give me some tips or advice on how to determine if the engine in my 72 Dart is the original powerplant that came with the car itself. I've read somewhere things like, look at the left side of the block and whatnot... but I am needing an "Explain it to me like I'm in 5th grade" kind of guidance LOL