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...I have done this conversion a couple of times, and I have no idea whether a DA 7400spindle will work for this. Nor do I know where you would buy an old DA 7400 loose ball spindle nowadays. I did mine by sorting through a bin of spindles at the bike co-op, and one worked.......out of maybe a hundred in the parts bin.
I think, but am not entirely sure, that the "7" spindles were more commonly found on the mountain bikes of that time.
You might also check out old loose ball Italian standard spindles, which are about the right length between the bearing shoulders, but I don't know what you'll run into in terms of the bearing curvatures.
Mostly, this is one of those things you just have to try out in dry mock ups until you find something that works.
I have heard of people using sealed unit BB's with nylon cups on both sides, and just forcing them into the Raleigh proprietary threads, because they are nylon and will deform somewhat to fit. I have never tried this myself, and while it sounds possible in theory, I wonder if it's a lasting solution.