An update based on further research. My Raleigh was built in Holland under license by Gazelle. According to what I've read elsewhere on the forum, those used standard 24 TPI. Curious, I measured my BB shell against a Nottingham built, '78 Mixte I'm fixing up (with cotterless cranks, oddly enough) -- my bike is definitely narrower, looks like less than 70 millimeters compared to ~71 millimeters for the Mixte (hard to be more precise with my measuring tape).
Does all this indicate I'm dealing with standard threading? If so, does it change the equation at all? That is, if I'm able to do a straightforward conversion to cotterless, is it worth it? Again, I should note I'm not actually experiencing any trouble with the cottered cranks. I'm mainly looking to do preventative maintenance and to scratch the everpresent upgrade itch.