dddd

I don't find that ISO taper is a problem with vintage campag cranks, at least not in terms of the engagement.
At worst, on a really stretched crankarm, one might have to grind a mm off of a spindle end.

But the bigger issue is whether you will find an ISO bb with the right length AND offset to work with your old cranks on your particular frame.

I'll add that Sugino Mighty and Suntour Superbe tapers are identical in width to ISO tapers as found on Campagnolo cartridge bottom brackets.

And I agree that ISO is closest to old French tapers compared to old Campag or especially compared to JIS (which is a bit more than .2mm thicker than ISO).

I tend not to worry about "taper length" as it has never been an issue for me (If the crankarm touches the inside flare where the taper ends, it typically happens after significant press-fit has already been achieved).

One more thing.
At most, the difference in engagement depth/travel due to mixing the biggest JIS taper with the smallest ISO taper will be <3mm on each side.
Phil used a hybrid JIS/Old-Campag taper that split the difference, so affected chainline by at most about 1mm this way or that. I have one here, measures 12.8mm instead of 12.7mm (old Campag) or 12.9mm (JIS).

For those with a caliper, comparisons between different tapers can be made as shown (JIS Dura Ace 7400 spindle shown).
Also, by locking the caliper jaws at say 13mm, one can measure how far up the taper that two different spindle tapers allow the caliper jaws to slide up the taper.
For numerically accurate (versus comparative) measurements, be sure to check/calibrate the caliper's measurement using a new ball bearing. Simply zeroing a cheap or worn caliper may result in undersized measurements.