From the chart with the gradient lines, it appears they are showing that the maximum difference in mounting position from the end of the spindle would be 2.5mm if going from the smallest (Ofmega) to the largest (JIS, i.e. Maxy).

But what they are trying to say at the bottom of the chart, "left" vs. "right", etc., makes no sense to me.

This is why I always keep an assortment of bb's/spindles around, even a bunch of dead ones, so that I can make relative measurements in real time with my work without interpreting anyone else's sizing opinions, and make test-fits on the fly.

To the OP, I would firstly evaluate where the existing cranks sit, if they are symmetrical about the bb at the pedals, and if the chainline is optimal for your setup. Then take it from there!
I would not hesitate to try to substiture a modern Shimano or Tange cartridge bottom bracket if the pedal symmetry and chainline are made happy.

My measurement for comparison of taper width is shown below, with the point of measurement from the spindle end being equal to the caliper jaw thickness or 1/8".
Locking the measurement and then transferring the caliper to a different spindle can tell you how much more/less that the spindle will slide into any given crank's square bore.