I assume by "the first cessna 180 generation being built" you mean a 1953-1956 "straight 180", as opposed to a 1958 180A or later?
FWIW the straight 180 serial numbers run from 30,000 through 32,999 and 50,000 through 50,105, the 180A starts at 50,106.
The K engine is listed on the TCDS for the straight 180, the R is not.
I've heard of many of them being installed in the early airplanes though.
A 2014 180/185 Club newsletter I have lists a copy of an STC for the "180 engine conversion to R" as being available for $4.
It seems like going from a K to an R would be a pretty simple field approval, if field approvals are available where you are.
Unless you're getting a great deal on an R, it'd be simpler yet to just stick with the K.
IMHO they're a good engine, I don't see much of any advantage to an R.
You could look through the STC's for the 180 via the FAA library website.
RGL Home (faa.gov)