The table on this page will give you the possible choices:
. Since its a K-10, it looks like 3.23 and 4.10 are possibilities. The table seems to say that 3.42 and 3.43 were only used with smaller engines, but I might be reading it wrong.
Many of these trucks don't have the ratio on the RPO sticker if it is the stock ratio. It may only be listed if it is an option. My truck had a metal tag on the rear end pumpkin, and you could use it to figure out the ratio. It did not say 4.10, but instead showed the number of teeth on the gears - something like 45/11.