What makes the truck rare? Were there a limited number made? My uncle has grandpa's 1930 AA grain truck in original shape. I'm not sure it's rare. It's old, but I don't know about rare. If it weren't touched (as in restored) it would increase the value.
The best advice is call your insurance agent and ask who he recommends as an antique auto appraiser. I have several antique vehicles and find the appraisers to be pretty spot on. I think you will be disappointed, but they will be accurate.