doesn't look to have been modified. with that being said, check the head numbers as mentioned above. this should get you close.
the displacement is cast into the back of the block. 4.8l and 5.3l engines shared the same block, different stokes. iirc, 4.8l pistons were flat and 5.3l pistons were dished. a bore scope camera, or a sharp eye will help you here.
6.0l engines had their own blocks.
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