Well now. The serial number block is blank.
I'm seeing a 6 (or maybe 5), a 3, a 5, maybe a 2, and an X in your picture. Get a wire brush and clean it up some more. Lighting it from the side also helps you read the stamped numbers. [on edit] on second look, that 2 looks like a 4, and a 54X indicates a 54 235 engine originally installed in a 3100. If you find an F in front of the 5, that would indicate it came from the Flint plant, and a T would indicate the Tonawanda Plant.
The casting number should be under the fuel pump and just above the pan gasket. Not that hard to find.
[on edit] Dan beat me to the punch while I was out double checking on my 216
(which is what your 54 should have if original) Nope 54's had a 235.
