When I was looking at this a good while back, for my '67 Newport bucket seats, it seems that the basic foam was used from '65-'67 model years. Those factory buckets were all shaped the same, but with different upholstery. 1968 seat backs were taller and had provisions for the federally mandated seat back lock mechanism, so they are different than the earlier models. BUT it could well be that the seat cushion foam "buns" are the same as the '67s.
I concur that Legendary should be where you look for these things. Might be that they will have some in the future? Do not look for Chrysler-specific items in this area, but C-body in general. In many cases, the foam would be common with the coverings being model-specific, sliding over the same foam shapes.
In general, the basic structure of the seats are the same and would build the same, regartdless of model year. Something that a good trim shop could do, IF such exists in your area. As strange as it might seem, look for the trim shops which current new car dealers send their warranty work to, for repairs or installation of factory covers. Those places should exhibit the best workmanship, but there could be others that also could. Reason? Who wants a new (or newer) car that looks like it's had upholstery work done?
As for the tech support people not knowing of model year cross-overs? They ALL go by what's in their catalogs ONLY. Usually no knowledge (OR anybody on staff that might know) of what might be similar enough to work. For example, the body mount rubber pucks on my '68 Buick LeSabre have "Chevy" casting numbers cast in to them (in a time when each GM division had their own individual number spectrum), with the same numbers listed in the Buick parts book for that year. BUT if you go to a vendor and ask if the similar Impala parts might fit a similar Buick? The answer will be a quick "NO". They'll claim that nothing will interchange, although there could be some "under-the-skin" parts that actually do.
Also be aware that in the world of factory parts, relating to seat foam, IF the factory supplies two parts, one with a slice to accommodate tie-down access to the seat frame, but one application doesn't need those, then that generates TWO different factory part numbers from the same original seat foam part.
End result, things might not be a bleak as you might suspect, or at least not quite as totally "blank" as originally perceived. The main difference will be the seat back, rather than the seat cushion, I suspect. Might just need some new overlay foam with some good upholstery to go back on top of it, which is "trim shop territory", usually.
Any seat from that era that uses a base of "zig-zag" springs as the basic support mechanism can be rebuilt, as the factory intended. The full-molded seat foam was generally used by GM, back then. Look underneath the seat and see if you can see the exposed seat springs, as mentioned.
Perhaps they changed something for '68 that I'm not aware of?
Just some thoughts,
CBODY67
