Ah! "Claw". This is the search term I need. Thanks!
In learning about derailleur hangers I realized this is an adjustment that has caused me annoyance in the past and that I would love to be able to fix it next time. And I am a tool collector anyways. So I already ordered that alignment tool.
Hard to get precise measurement between the dropouts without taking the wheel out, which I don't want to do this morning. But it seems to be around 135 mm. Sheldon Brown says current common freewheels and cassettes are also around 135mm. It doesn't look like I will have any trouble with frame width. Not like when I wanted to upgrade my 1980s 10-speed.