What's the small print at the very bottom of the card? I can't really read it well at any resolution. If it's important, it should probably be noted in the description eventually since it's not legible.
Is the goal here to archive these cards for posterity's sake, since they are out of print and likely to become scarcer and scarcer? Or is the goal merely to just document them for the reference? If we're looking to archive, then I'd say go with 400 pixels. If we're looking to document, then I'd lean towards 300 pixels but 223 could also work.
If we get asked to pull them down for being too good, we can probably just negotiate a downsampled size at that time instead. If they notice the 400 pixel images and complain, then at that point we can use a script to downsample them all to 300 or 233. It should be fairly straightforward to automate that, if the need arises.
Is it possible to scan them in as PNG instead of JPG? Given the type of images these are, they will probably store well as PNGs, and PNG is lossless which means they'll rescale without gaining artifacts. If we're just documenting, then it's not really a big deal; but if we're archiving, then having them in a lossless format is preferable.