Do you have a title to the truck? What number is on the title? does it match the number stamped in the block?
There is good info here on decoding your vin number.
http://www.stovebolt.com/techtips/series.htm
If you do have a tag on the door post with a number stamped on it don't remove it no matter how beat up it is as long as you can read the number. I've seen a lot of them that the old slap and dash paint shops painted right over in the 60's and 70's. It doesn't matter if it is painted over as long as it is there with the original rivets.