Having just gone through the 7401 to 7403 swap, I can tell you that 7403 freehubs are basically unobtainium. I got very lucky and picked up a set of 7400 hubs with a 7403 freehub off ebay. The special tool to remove the freehubs from the hub body is required. After removing 2 of them, I can say with certainty that there is no way to remove them without the tool.
I swapped the 7403 for the 7401 so that I can use a 10 speed cassette, and it does work and fits properly. The 7403 freehub has both internal threads (for a lockring) and external threads (for a threaded cog).
If you do get a 7403, take good care of it, they are very hard to find.
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Dale, NL4T