These are what my friend used. He found them off craigs list years ago. I would imagine anything from 99-07 classic would work since I have swapped the 1/2 tone mirrors between all these years. Between he and I driving it, we've never actually needed to extend them. Only trailor we pull though is an open flatbed car hauler.
They are WAY better than the factory ones that came on the 1-ton.
This place still lists them, bout 150 a pair
http://monsterautoparts.com/CHEVROLE..._mirrors_3.htm