Robert Anton Wilson biographer Prop Anon points out an avenue of research on Twitter.
Prop writes, "Robert Anton Wilson wrote for FACT magazine in the early 1960s, long before Illuminatus! This website has ALL the issues of Fact up and its free to peruse."
The magazine was edited and published by Ralph Ginsburg; the first issue at the website has a piece about the drug experiences of "advertising man" Ronald Weston, a pen name that RAW used.
The magazine lasted for three years, 1964 to 1967; Wikipedia has a short article.