John Gunnell's book "Chevrolet Pickups 1946-1972" has US production numbers starting with the 1965:
Chevy Van 42,999
C1400 (C10) 131,395
C1500 (C-10) 186,461
K1400 (K-10) 2,788
K1500 (K-10) 2,024
C2500 (C-20) 64,282
K2500 (K-20) 2,561
Total: 432,510

Only 1.70% were 4WD in '65. In '66 it's 1.83%. Back then, people who bought 4WD needed it and they tended to get hard use. My Dad bought my '64 K1404 for cutting firewood in northern New Mexico. It saw a lot of miles on Forest Service roads but the road salt is what did the worst damage.

Last edited by Big Tan; 12/27/2008 1:28 PM. Reason: Clarification: US production