A little update... Originally none of my 12 leaf spring grease joints would take grease. All the fittings were fine (and those parts I listed above are the correct parts though), so it was the actual joint that was stuck shut. I'm guessing that at some point, decades ago, the truck stopped taking grease, and the mechanics just gave up trying. I put a whole tube of grease into the u-joint and drive shafts when I first got it, if that's any indication how seriously they took greasing.
For the stuck leaf springs, I decided to try an IPA Grease Joint Rejuvenator, $50 at Tractor Supply.
http://www.ipatools.com/products/ipa-7862.html
It's basically a metal syringe that you fill with penetrating oil and hit with a hammer, to forcefully drive lubricant into the joint. That thing was able to get about 1/2 of them unstuck, before I destroyed the 2 zerk joint couplers that it came with. I've now ordered 6 extra couplers (
http://www.ipatools.com/products/ipa-7902.html ), which arrived today, and I'm hoping that I can unstick the other joints before they're destroyed. Pretty neat tool overall.
If I still can't get them unstuck, I'm wondering if I can't design a similar but bigger, heavier duty tool that will. Or perhaps make an adapter for this one that lets it screw right into the grease zerk's threads (with the grease zerk removed), so that I can beat on it as hard as I need to without the couplers being the weak link.