True Raleigh bottom bracket cups were the same diameter as English threading but were threaded 26tpi instead of the standard English 24tpi. I'd be very surprised if Nishiki used Raleigh's threading.

Also, you said your bike was a Nishiki "Olympic 12". That implies it came with a 6-speed freewheel. Usually that means 126 mm dropouts but it may have been equipped with a Sun Tour "Ultra 6" freewheel which worked with 120 mm spacing. That dates the bike to late 70's or very early 80's.

I'd go with a Shimano cartridge bb if the threading really is English. The non-drive side cup has no flange and can be screwed in as far as needed to hold the bb in place so the extra 2 mm won't matter.