Fintails, /8 and Mercedes from that era used 185HR14 radials which is no longer available. The cloest equivalent is 185/80R14. These are difficult to find at best and people use 195/75R14 or 205/70R14 - but with 205's you need 6.5" rims, not 6" rims. An ever closer match is 215/70R14 - that's within 0.07% of the original size. See this chart.
As far as rolling diameter is concerned you are correct. However if you go
and talk to any suspension guy who knows his stuff he will tell you about
how each tyre has its ideal rim width. Not surprisingly the MB engineers
also has a fare idea about the relationship between tyre width and rim
width.
W115 240D 5.5 inch rim with 175 tyre
W116 280SE 6inch rim with 185 tyre
W116 450SE 6.5 inch rim with 205 tyre
Etc, etc.
A 215 tyre would need a 7 inch rim to perform to its optimum.
