Quite so, my bad.
Technically, the Cathode Ray Tube itself has no specific resolution. It's the analog board and driver circuitry that sets it. In *theory* you could drive the tube at near any resolution - if you're up for designing and building your own high-voltage, high-frequency, precisely tuned electromagnetic tube driver...
Part of the reason I advised - and still advise - extensive searching and reading, is that this is *not even remotely* a simple plug-A-into-B hack. It's not something I would recommend unless you know what you're getting yourself into (I mentioned the high voltage, right?) - and not knowing the relevant search terms to bring up what you're looking for is not a promising start.
For example, no-one's even mentioned refresh rates, sync rates, or video and sync signal polarity yet. Sync is your biggest issue, IMO.
I would suggest starting with the comments I left on the very first, stickied, post ("What this forum is about") in the resurrected Hacks forum, where I dropped in a list of useful existing threads. Also, search term: eviltim
Oy vey. The original attempt at the PMGS involved transplanting the analog board and deflection coils from another monitor onto the SE's tube. Even he ended up using a complete substitute monitor in the end.
It's back up, btw, as part of the resurrected Power Color Classic pages.
Sync and rez aside (*snerk*), it's possible to get grays from a mono monitor by hijacking the brightness knob's PCB points - as eviltim/viletim demonstrated on one of his RGB monitor hacks.
Space wise, an LCD would free up a metric shed-load...