Another simplier way to check, is to remove one of the center freeze plugs on the block. Just inside the hole will be a stamp "428", if it's a 428. All 428's were stamped like this. If it's a 390, there will be no stamp. Of course you will need to have the block out of the car to check this, as it will be quite hard I think to check in the car. As one of the poster above said, you can't trust the block numbers all the time.