Older motors that do not have a partial VIN stamp that matches your vehicle identification number probably had the older engine yanked at some time. These cars were never kept numbers matching originals by the little old ladies and single mom's that owned them. When they broke down it was often cheaper to swap a junkyard motor, than a cam, or cracked head (Garages charge by the hour not by the part so if it took less time to fix by yanking a motor than rebuilding one in a stall it got yanked).
Whenever you are talking junkyard motor you are talking a 350 small block because there are more of them in the world than any other engine (therefore it was the cheapest engine to throw in there). Do not be surprised or upset if it has a 350 unless you bought it as a number matching original and paid for it.