Welcome to the board.
The info you have there on the 50th annv. isn't too acurate. The custom camper package was just an option package on top of any truck. The 50th was a gold and white 2 tone paint, and gold and white int. The truck had to be a Custom model, or a CST.
The GMC had no special that year...they were already older then 50, since GMC was founded in 1903.
Your truck, does it have the added 6 inch extention on the front of the bed? If so, anything refered to as a longhorn is the same animal....well, cousins anyways. For info on that, (if that is indeed what you have) then click on the link in my signiture for my Longhorn Website.
You will not find production numbers for your truck, just estimates. GMC numbers were lost, supposedly in a fire...but I'm sure there are a million stories on that.
Again, asuming you are talking about the 8 1/2 foot long fleet side bed GMC, and it is a 68, then you have the lowest production number of all the years, since it was started in mid production. (march to be exact) If it was a one ton, then it'd be one of even less.
If I am remembering correctly, i think it was one GMC for every 3 chevy trucks....that is the generic rule of thumb on that.
Welcome to the board, you have landed on the best site for these trucks.
Where arte you located...and as said before...pics are required.
(not really, but we dig pics)