Something has to be totally off... the fans for the 240 are 25mm thick. Those for the 280 are 27mm thick. The radiator itself is 38mm. So in total that's either about 63mm or 65mm - still you, however you managed it, make that a thickness of 69.9mm... and I cannot wrap my head around how you got that measurement.
So I can only assume that the data that you are supplying in your list of specifications is wrong or... well, it must be wrong.
What's up with that? How thick is it actually with the fans assembled to the radiator?
Slight discrepancies in imperial to metric conversions and rounding. The official spec sheet is here:
https://www2.arctic.ac/liquidfreeze...pec_Sheet_Liquid_Freezer_II_280_EN_LowRes.pdf
Which indicates that it
should be 65mm by adding the fan and rad thickness - howeverthese could be rounded to the nearest whole number as well. For example, it could 65.3mm + 27.3mm which could technically round down as standalones but should add to 65.6, which if rounded at this point should be 66, not 65. This is just a speculation though. I assume since this was mfgd in a metric country the mm indicated on the spec sheet should be pretty exact.
Also the screws that hold the fans to the rad stand off from the surface at least two mm - maybe three. this additional number is not fan or rad width but would definitely add to the overall package width. You can see the screw offset below.
The 69.9 mentioned in this article is an
exact conversion of 2.75
inches (and then rounded up to the nearest 10th mm) - in which the initial imperial value would have been rounded to the nearest .125 inches to keep with imperial measurement expectations for the USA - and then converted back to mm from there, which made the discrepancy even larger. Go with the manufacturers mm measurement, which is 65mm total and add at least two mm for the screw head offset.