Pretty sure the c10 comes with better gearing than the c20. Every '65 c20 I've seen has had the 4:57 gears. The front suspension has rubber bushings on the c10, the c20 has steel bushings. The rear cab mounts are different. The c10 has donut style mounts, the c20 has donut style front cab mounts and shear style rear. Aside from the heavy duty frame and running gear of the c20, the c10 is a pretty close match. They share the same rear trailing arms, bed, cab, fenders, etc. It's just the under carriages that differ.
