I've got a strong feeling that the 7400 bottom bracket used ISO tapers, and the UN54 will have JIS. There's contradictory information about this on the web, but generally the sources I trust lean towards ISO for the 7400 models, with 7410 being the shift to JIS.
the 7400 axle was also asymmetric.
EDIT - here's Andrew Muzi, writing five years ago:"The asymmetric 1046a Nuovo Record BB assembly has an
identical spindle with the Shimano BB7400 for the first New
Dura Ace. That pattern is correct to another dozen cranks
of top quality in that era. Or an original Phil Wood #2 BB.
That was a de facto standard among professional quality
bikes for over twenty years, across nearly every major
brand. (OK, not the two big French houses but they were
always intractable on purpose)"https://groups.google.com/group/rec.bic ... 160f0c7230
Interesting, I just assumed that Shimano would obviously be JIS
No real issue though as I've used JIS cranks with ISO b/bs and vice versa in the past with no issues and should be even less of an issue with a sealed b/b when there isn't the need to remove the cranks to service the b/b - I think it's the regular crank removal that messes up the taper rather than the interface itself. Not ideal admittedly, but should be fine in the real world.
The asymmetry sounds more of a concern but the judicious use of a narrow spacer or two should address that - I'm assuming that the axle needs to be longer on the drive side and if that's not the case it could be a little trickier.
Compatability - you've got to love it