JR,
I checked my books as I mentioned I would and got re-reminded that GMC Numbers are different from Chevrolet. Even though I've always liked both, for some reason I end up with Chevrolet books. Although I did find out a little info on it. The HM 5500 is listed in one of my books that it was made for 2 years only, in '66 and '67. Therefore it looks like your truck is a '66 as there is no mention of an HM 5500 being made in '65.
Bill
