According to the shop manual, 1965 Supplement ST-61, Specifications 11(page), Fuel Tank(section 8) :
Cab Models:
C L 10-60
M-60
K 10, K 20
Std. / 18.5 (Gallons)
RPO NO.1 / 21.0 (Gallons)
Panel and Carryall models:
C 10, K10 / Std. / 20.5 (Gallons)
C 30 / Std ./ 18.5 (Gallons)
Guide to '60-'66 Chevy Truck Manuals - tech tip here on the bolt 
