A friend of mine bought a 61 bubble top Impala with a 348. When the car was delivered it had an X on the block. I told him it was a 409 and really between the both of us--we're really not experts. When I read the code TF, it indicates a 348 truck motor. I thought only 409's had the X, but what do I know. Please give me a little advise on this. Thanks.