"AFAIK, Sugino doesn’t use a proprietary taper, unless that’s what you consider JIS to be." No, I do not consider JIS to be proprietary, however Sugino's online store lists a number of cranksets that use a "SG Mighty 4-Axle Taper" spindle; are you saying that this is only a fancy marketing name for JIS?

"An early Klein to me would be 1975." Please excuse my faux pas

Phil Wood External Bearing Kit - the problem with this is that there really aren't any modern crank axles that would span a 76mm BB shell + external bearings and where I live I am never going to find anyone who could face off the shell to a more normal 68mm

"If you are OK with the current (square taper) bottom bracket axle, could you take some digital calipers & let us know what the BB axle width is (mm width)?" it's a 119mm spindle running in press fit bearings in a 76mm wide shell