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...the only people I know of who make a BB unit in proprietary Raleigh 26 tpi threading are the ones at Phil Wood, which is pretty expensive for a three speed conversion.
I have heard from multiple sources that people re-tap these to standard 24 tpi all the time without any problems down the road, but I try not to do stuff like that myself.
If you decide to go that route, the Shimano units marked for 68 mm shells will work, you just have to tighten the lipless, non drive side cup in a little farther, to lock the cartridge in place. Not hard to do.
You might have to fiddle around a little bit figuring out what spindle length you need, depending on you crank you pick. the problem with the method you read about using a replacement spindle with your present cups is that those spindles can be hard to find these days. It used to be an easy swap when there were more of them around in the parts stream.