It's no trick to identify a side oiler block. Just look for the "hump" in the driver's side of the block. That's the oiling passage that makes it a side oiler. Also all 427 blocks S/O or not, have crossbolt main bearing caps with the "crossbolts" (center 3 caps only) going through the block just above the
oil pan rails.
Now, that said, there have been some 390-406-428 engines "converted" to crossbolt mains by owners, not by Ford. There have even been some fakes made, though for what reason I can't fathom.
If the engine is sans heads, measure the bore. Stock 427 is 4.23".
If I were you FUNFER, I'd do some quick inspection. If the block is not obviously broken, I'd pay the man his $300, load the parts and run like hell before he changes his mind!
Al