Turn the headlights on. If they're nice and bright, you should have plenty of juice all around. Sometimes those replacement, bolt-on terminal clamps just won't do it and you gotta break down and buy some proper wires.
My experience is that these engines can lose compression pretty fast. I've been in situations where: It started fine last month. I know it's got compression! And lo and behold ....Check compression. Of course, you gotta have plenty of juice to the starter to do that ....
And if you've been checking plugs and distributor a lot, remember how easy it is to "replace" the wires to the wrong cylinders. I've, ah, done that a time or two.

'63 half-ton
another '63 half-ton
'62 school bus