I don't have a book with 351W casting numbers that new but it's an 1990 model year block, cast on September 14, 1989 More than likely came out of a pickup or full size van. Some were put into full size Ford's,Merc's,& Lincolns, but the majority were put into trucks and vans. Sounds like someone did a bit of junkyard swapping to put it into the 69 Stang. Look at the heads and see if there's a large "S" or "T" cast into one corner, next to the valve cover. "S" heads are E6SE's, high swirl/torque heads. "T"'s are the better ( by 25 horses on the top end) Stang/Pickup/Van E7TE heads. No letter there? Then look inside the valve cover and see what the date code is there, and/or look under the intake runners and get the casting numbers there to see what heads they are.