how do i measure the stroke

[Pumkin Truck]

to tell the truth, the engine is a cad i know for sure, but the person who i got it from was not sure to what year or model he got it out of. the 472 and 500 have the same eternal dimensions and have been told the only difference is the stroke, so i am crossing my fingers hoping it is a 500, i couldnt find the block numbers so i am gonna try and atempt to do it the hard way, but i dont know how

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1969 Gmc swb stepside Wimpy six soon to be a 500cid cadillac!!