Lower-end bikes in the old days very typically used over-length spindles so that beginners who ended up riding with both levers slammed forward would not encounter noisy transmission.
It was fairly typical for a percentage of beginners to ride their road bikes this way at all times, since the grinding noises of shifting signaled to them that they were doing something wrong, or because the friction adjustment screws had lost tension after some period of service.
Many such riders were loath to ride one-handed even for the time it took to friction-shift gears!

While I myself (for my own use) have changed many such spindles to a shorter length (or an effectively shorter length via a symmetric cartridge bb), I can understand why the bike makers in those days did what they did, and why someone (as the case here) might want to do the same.
Many of us here are old roadies, who by habit use the big ring for 90% of their riding, but it seems this bike's rider likely presents the opposite case.

So a longer spindle can be used, just a millimeter or two will compensate for the chain angling of perhaps a full two or three cog spacings in back since the longer crank chainline is far more local to the critical position along the length of the chain's upper and lower runs than is the cassette!

Crank bearings with space for 11 balls all have the same bearing track diameter, and differences in the load path angle is in all cases plenty close enough for smooth running, as smooth as the bearing surfaces are in any case. So find a spindle with same ISO taper as the original, or use a length compensation of 2.5mm shorter off of each end to use a JIS spindle, and add a millimeter to the driveside end. Can you not just add a 1mm spacer under the driveside cup flange?
You will need to get the final driveside spindle projection length correct by whatever means, and will have to use a spindle with the same width between races, but you seem up for this bit of experimentation. Better yet, try a couple of cheap cartridge bb's and perhaps save considerable time!
Lastly you will want to apply a very thick chainstay covering and be certain your derailer is tensioning the chain sufficiently.

I agree there are riders who just have no interest in using the big ring, I've repaired bikes where all riding was done with both levers slammed forward and have even seen the small threaded cog split open from such use. But it took a good while for this to happen, and the rider was obviously getting plenty of use from their bike.


I would consider putting a thumb shifter on the top of the bars for such a rider to at least feel safe while acquainting themself with the shifting process, but there is some value in letting a rider learn the art of friction shifting instead of dumbing the process down for them completely with a click-shifter.