You could put all the common headers in a property drawer of top-level heading, and all child source code blocks inside that heading will inherit them.
* Notebook
:PROPERTIES:
:header-args:R: :colnames yes
:END:
#+begin_src R
words <- tolower(scan("intro.org", what="", na.strings=c("|",":")))
t(sort(table(words[nchar(words) > 3]), decreasing=TRUE)[1:10])
#+end_src
As for making the block delimiter look different, there are various ways of making the text display differently from the actual text. For example, you could use the built-in `prettify-symbol-mode` with this config:
