Some more info on frame lengths for 3100s for various years. Data from Vehicle Information Kits (available HERE)

1955 2nd Series - 178.05"
1956 - 178.05"
1957 - 178.05"
1958 - 180.55"
1959 - 180.55"
1960 (C14) - 180.0"
1961 (C14) - 180.0"

So it looks like you need to narrow your search to 1958 - 1959 trucks.

Kevin
1951 Chevy 3100 work truck
Follow this saga in Project Journal
Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.