Thanks, Mark. Based on your comments, I tried the vice-and-socket routine again and this time I was able to get one of the cups free -- from the shaft-side yoke. But that was it, so out came the cut-off wheel (fortunately, the spider had shifted far enough to one aside to let me get the cut-off disc into the gap without damaging the inner face of the yoke). After using the cut-off wheel, I was able to separate the shaft from the splined yoke, which made subsequent work a bit easier.
I had to cut both legs off the spider to get it out of the splined yoke. Even after I'd extracted the remains, there still was one cup that refused to budge more than about 1/32" in either direction. Sixty seconds with a MAP torch got the yoke hot enough so I could tap out the cup with a punch.
Additional comments for anyone else trying this:
For each of the two yokes, I found that one of the two cups would not move inward (and, yes, all the retaining clips had been removed). That meant that my press-out efforts with the vise and sockets would only work in one direction. Even in the successful direction, there was a loud 'crack' before the cups started to move.
Instead of using an oversize socket on the exit side of each yoke, I ended up grabbing some 1x2 hardwood scrap and drilling a hole in it (11/16" drill, if you do this). Thew wood piece could then be propped up inside the vise jaws to the right height and then sit there without needing a hand to hold it in position. It was then much easier to juggle the combination of the other socket, the steering shaft and the vise handle while trying to get everything aligned and the vise jaws tightened down enough to hold everything in place.
I think I'm going deep-freeze the new spiders and cups and then use a torch to get some heat into the yokes before I try re-assembly with my new Kawasaki U-joint kit. The cups look like they'll be really easy to get cocked a bit sideways at the start and I don't want to risk chewing up the bores.