I'd guess 5,000-6,000 on the weight. Go by the dump or scrapyard and roll over the scales to see what ya got.
I'd consider just using the 18" lock ring wheels if you don't have a problem with the available tire selection (or lack thereof). I know a lot of places are weird about it, but that's why I mount my own tires, lol. A C-40 is the standard 8 on 6.5" pattern, right? If so, some later common-as-dirt 16" tubeless wheels should slip right on if the brake drums clear, although I kind of doubt they will on a C-40. (Someone else help me on this) Also shorter than the 18" tube types. (=more engine speed)
19.5" wheels should clear fine, and they look really nice, IMO. You also get "real" truck tires with the swap; the stability and tread wear is supposed to be great. Tires are kinda spendy, and the wheels aren't that common, though.
Here's a link to a thread on another site where you can see the 16" and 19.5"s on a C-30.
http://67-72chevytrucks.com/vboard/showthread.php?p=4547831
