Neil Sinhababu and Nate Silver have to say on the subject, it's worth pointing out that Gillibrand's voting record in the Senate puts her firmly in the leftmost third of Democratic Senators. This was all fairly predictable at the time of her appointment. While a member of the House of Representatives, Gillibrand tilted slightly to the left of her right-leaning Upstate constituency. Given a more solid Democratic base, she proved to be a more-or-less mainstream liberal. DW-NOMINATE places here on the left-right spectrum as the 10th-most liberal Senator, tied with Frank Lautenburg (D-NJ) and sandwiched between Roland Burris and Patty Murray. This is more-or-less what you'd want out of a New York Senator. Indeed, her record is slightly more liberal than Hillary Clinton's, so the decision to make Clinton the Secretary of State has actually shifted the Senate a bit to the left.
It's entirely unclear to me what sort of political coalition Harold Ford hopes to build against her. I guess you figure that she'll win Upstate and liberal women, and Ford would counter by trying to rack up big margins among African-Americans and ... outer-borough and exurban "conservative" WMDs (White Male Democrats)? Puerto Ricans and Dominicans? Jews? Union rank-and-file? How is this supposed to work?
Meanwhile, though there are many reasons Ford is ill-suited to represent New York, "being a Southerner" isn't really one of them. "Being a transparent panderer, particularly for positions well to the right of the median New Yorker", certainly; but there's no need to hold the man's accent against him.