Hi SCJohnson,
Welcome to the VCCA's forums website and to the wonderful world of old Chevys!
If you haven't yet purchased a shop manual, I highly suggest that you get one. They are very informative and will help you keep the old gal running right. ;-) There are reprints available on ebay and also from vendors like the Filling Station.
According to the Car Number plate number you provided, you do indeed have a '28! The LO (not L zero) means it was produced in the first half of 1928. If it were a '27, the letters would be LM. The trucks produced in the last half of 1928 were model LP with four wheel brakes.
The 21 means that the truck was built in the Janesville, WI plant. The 1207 means that it was the 206th vehicle off that assembly line for that year (the first vehicle would be number 1001).
Your truck should have a horn button in the middle of the steering wheel. I cannot help you with the wiring because my '27 is not correct and the wiring is functional, but does not reflect the original.
Same with the exhaust pipe length. There are other experts on this site, however that can answer those questions.
Since this is the New User Forum, I suggest that you pose your technical questions in the 1912-1928 forum or for the wiring, in the 6V electrical forum.
The more information that you can provide about your truck, the clearer the answers will be.
We'd LOVE to see some photos! If the photos reside online somewhere, you can insert an image in your post using the Full Editor and clicking on the Insert Image icon.
Cheers, Dean