As I recall, in my 1985 300D, they ride "just right" like posts 11 & 13, not "too high" like post 5, or "sunken" like post 9, though all probably work fine. Main thing is that the two are matched, though even that doesn't matter if they follow different routes like some engines in post 11. Rock lists AC Delco 15405 (10 x 1029 mm), Gates 7405 (10 x 1029 mm), Dayco 15405 (11 x 1029 mm), and Continental 15411 (11.5 x 1044 mm). The lengths are "effective length", which is 1.5% less than "outer circumference". Unsure about Continental since they spec as just "Belt Length". I found various manufacturers spec lengths differently (not always defined), when I sourced a belt for my 1965 Dart, where length was critical since limited adjustment in a custom AC bracket I fab'ed. Went thru 3 belts before I found a good fit. Trying to estimate length from a rope measurement made it tricky.

It looks like the Dayco and Continental belts may ride a bit high since wider. Strange since those 7/16" and 15/32" widths, resp., don't appear to be standard. A Gates catalog I downloaded has belts of 3/8" and 1/2" widths, with an oddball at 13/32" (states 9.5/10 mm in parenthesis, though 10.3 mm). That range suggests anything from 9.5 to 11.5 mm width is considered interchangeable, as does Rock's listings.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans