I'm not sure if your truck plate is similar in layout to the passenger car protect o plates. You will be able to judge.
Here is an example of a 1968 Camaro plate with an explanation of the various numbers.
http://www.holisticpage.com/camaro/parts/68protec.htm Apparently the answers to interpreting your plate are found in various GM service manuals, assembly manuals or bulletins used for your truck. You probably know that there are a number of vendors who sell reproduction manuals for your truck year.
If you're ok with other people's best guesses, here's mine.
05 don't know
R Carb source, I believe. Rochester
HA0613K is the original rear axle ID code.
HA 3.73 Rear end
06 June
13 day of month
K McKinnon Industries
6 not sure
S8H11 is the original transmission ID code
S possibly Saginaw 3 speed
8 1968
H June
11 day of month
This site has tips for identifying chevy manual transmissions.
http://www.nastyz28.com/chevy-manual-transmission-identification.php I'm not endorsing this vendor but here are several books that you may decide to purchase or borrow to help decode your plate. (I don't own any of them nor do I have a relationship with the vendor)
http://www.themotorbookstore.com/gemoprdegu19.html http://www.themotorbookstore.com/capacaofchtr.html http://www.themotorbookstore.com/67chevytruck.html
