Which is correct? My gut tells me it's the second one, but I've asked and someone told me the first is probably the one to use. Reasons being ポータル is not a common katakana word.
Neither is incorrect, but I would say 日系アメリカ人の歴史についてのポータル(サイト) if there is enough space. (This ポータル means a web portal, right?)
The first one has two simple の's in succession, which is not bad but may be seen as unsophisticated. (Three or more の's is seen as clumsy by many.) The second one treats 歴史ポータル like an established set phrase, but I don't think it is a well-known established concept. Although these are minor problems, it's better to avoid them when possible.