I don't know if replacement blocks have have some sort of different marking, but I'm attaching a picture of my 1969 C9OE block and circled where the casting # is.
I have never seen any serial number or VIN-related thing on the 3 SBFs I have in my shop, but I wasn't especially looking for them. Now my personal opinion: I in absolutely no way condone ever touching stolen property. But... I doubt that there are a ton (any) police looking for some stolen 351 motor, especially if it's in a Factory Five car. On the one-in-1000000 that that motor is actually stolen property, it happened so long ago that is has probably been forgotten.
Just my $.02 though...
'94 Laser red 'vert. Still a 5-oh... for now