Back to the question of year (anecdotes from states other than the OP's, while possibly interesting, are not helpful. Only his own state DMV can tell him what he needs to do) ...
We need to see pictures of the cab to tell you year.
Or you could try this, for starters Yes, yes, I know its for Chevrolets (I wrote it
) but it mostly works for GMC's, too.
Turn down door handles are on 47 through 51. Its only a '51 if it also has the vent windows in the door (vice the one-piece glass) and no side cowl vent (replaced by the vent windows). Unless the doors got changed ... Pictures will help the GMC experts id the year for you.
