I'm betting that what really happened goes something like this: a body shop took a smashed up or used up V6 that was purchased on the cheap, gave it a hood and paint, stuffed a 289 they had laying around into the engine bay and, i'm willing to bet, attached it to the V6 tranny/rearend.
then said man/child/fool who knows little/nothing about cars, let alone mustangs (we'll call him the 'chump') purchases a "fast" car at the recommendation of shop staff while bringing his car in for repairs, and they laugh all the way to the bank because i'm sure they charged him somewhere near that reserve price. OWNED!