No evidence has been found to provide any confirmation of the building being older than 1600, no evidence has been found to corroborate a pre-Columbian development either, however considerable evidence has been found to support the contention that the building {and the site} was constructed in the mid-1600's thus far. As you note there is a second season of digs due to start now, with detailed results due in 2008/09. It's also worth noting that when Menzies uses an academic source to question, for example, the validity of tests re the structure, he 'forgets' to mention that the paper referenced offers a detailed argument for Norse construction {though this too has no confirming evidence at this time either}.
However, as I originally noted this is a single element, one could, for example, search for discussion re Antilia and the point that the map shown in the book has been spun from the near vertical to the horizontal try to resemble Puerto Rico.
Ultimately, the hypothesis requires considerably better evidence than is furnished in the book. I recall the late Carl Sagan's quote, 'Extraordinary claims require extraordinary evidence'. By the by, this was rebutted with the comment that such claims* required 'extraordinary investigation', to which one can only respond, as did he, yes but in a spirit true to science.
Regards
*about UFO's and abductions