Now the end game. On one level it seems a bit silly to post every little thing, but on another it might be useful to somebody else with the same issue (and I'm betting there will be somebody). So, here's the conclusion.
> This job was pretty straightforward. I never take that for granted!
> All the vacuum lines checked out, no leaks. Rubber connectors are soft and supple (obviously changed at some point before they were all NLA, I'm amazed). Heat levers worked as they should.
> The original part for the 6.9 (and 450 in general) is apparently NLA. MB has a substitute but it's not 100% identical and won't fit flush against the firewall. This is easily fixed by using a couple 1/4" spacers between the valve and firewall.
> Since the grill under with wipers has to come off, it's a great chance to check all the dirt and crap that gets underneath and clean out the drains.
> At the outset I didn't have a vacuum pump. Having one would have saved me a lot of time. Doesn't have to be big, a small one is still useful and not very expensive ($99 from Harbor Freight).
I must confess, all the vacuum stuff had me kind of intimidated. I'm used to working with electricity! But the logic is pretty simple, as SquiggleDog points out. Now that I can run the engine again it's on to the next project - putting suspension back together. More on that in a few days. thanks to all who helped me through this. Your expertise will be tested when I get to my strange WUR problem.