Difference between C10 & C20 (1970)?

[72lb4x4]

Basically, its load carrying capacity. 10 is 1/2 ton (1000lb) and 20 is 3/4 ton (1500lb) (30 is 1 ton). Suspension parts are more heavy duty on the 3/4 and 1 ton than 1/2 ton. Frames in some cases (C30) can be beefier too I think.

3/4 and 1 tons will have larger tires and 8 lug wheels.
Cab mounts seem to be different but I'm no expert there. Just basing this from info in catalogs, since mines a 4x4 and they seem to share mounts with a 3/4 ton.

Shortbeds came only in a 1/2 ton version as far as I know, so if you want a shortbed, it may not be a good idea to get a C20 LWB and plan to shorten it.

Similarly, if you want to lower a truck it makes no sense (to me) to get a C20/C30.