Just aquired a good running FE long block. The heads are easily identifiable ( C7AE-A ) but all I can find on the block is the casting date ( 3C21). If I am correct that stands for March 21 1963 or could it be 1973? The block has raised ribs along the bottom of the main body running top to bottom. Also on the passenger side of the block there are Two threaded bolt holes just above the oil pan between #1 & #2 cylinders and between #3 & #4 cylinders.Could this be a service or truck block? All help is appreciated.

The external ribbed blocks are later blocks, likely a '73. The heads are regular heads. The holes are to bolt a factory fixture to for assembly. And yes it's a truck block as by the 70's only trucks got the FE. Most likely a 390 block.
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Larry Hampton
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'63 Fairlane 2-dr post. Race car since at least '67. In the process of turning it into a proto-clone Nostalgia Super Stock car with FE power and three pedals.

Thank you very much, I have several other FE's but this one is just different and I wasn't sure what I was kooking at.