cleedustee

I recently bought me a 1972 f100 stepside. The VIN tag says it came with a 390ci motor. The guy I got it from says he thinks it's a 351ci and with all the research I've done trying to identify this thing, I'm pretty sure it's a 351c. But here's the thing... The block stamp is "D1AE". How could it be a '71 351 if the 351 didn't come out till '73 or whatever??? I'm very confused. What's another way to identify this motor so I can get parts?? Please help.