You sure it's exactly the same? I did my ball joints/bushings on my '71 C20, then helped a friend do his '72 C10...and noticed the half-ton (C10) ball joints were smaller in diameter. They look identical as far as the taper of the joint, but the diameter was definitely smaller.
Other than that, another problem could be you can't get the spindle up high enough. Usually it's a problem if there's no engine/trans in the truck (not enough weight to compress the spring with a jack under the lower A-arm). If you have a spring compressor still on the spring, you should be able to get the ball joint in.