One hurdle in going with a modern bb/crank on those older Raleigh's with cottered cranks is they tended to use Proprietary threading on the bb shell and cups....ask me how I know
A solution to that is to go with a threadless bb like Velo Orange sells but that is $$$ and will blow any resale budget, more of a
I am keeping this for myself kind of a purchase.
Nice job on the paint and decals - thought the down tube decals looked fine btw. . I rattle canned an old Azuki that I had stripped down to turn into a single speed, it didn't come out half bad and certainly improved the look of the bike. These days I am a clean it up as good as you can and love the patina kind of guy but for a Grand Prix level bike I think a rattle can is perfectly acceptable.