6th digit will be a 3 for a 1 ton, a 2 for a 3/4. I would add though that there isn't a lot of difference between the 2 except for the front axle rating. If it's a 3/4 with a 4000# axle it's as good as a 1 ton. If it has the original options sticker under the hood or in the glove box it will tell you. Most 3/4 tons had the 3800# front axle but there was also a 3600# I believe.