Align arrow to text box with Tikz?

I'm new to TikZ, and I can't seem to figure out how to align lines to a text box.

In the figure below, the bottom arrow is aligned exactly how I would like (from the upper right corner of box C to the bottom left corner of box I. When I create a text box that contains greater than 1 character, the arrow becomes misaligned as in the top arrow.

How can I get the beginning of the line aligned to variable sized boxes?

Here is my current code:

\begin{figure}

\tikzstyle{vspecies}=[rectangle, minimum size=0.5cm,draw=black,fill=white]
\begin{tikzpicture}[auto, outer sep=1pt, node distance=2cm,>=latex']

\node [vspecies] (I) {I} ;
\node [vspecies, above left of = I] (S) {SFs} ;
\node [vspecies, below left of = I] (C) {C} ;
\draw [->,thick] (S) --  node {\small{$63$}} (I) ;
\draw [->,thick] (C) --  node [below right] {\small{$733$}} (I) ;

\end{tikzpicture}

\end{figure}


Simply refer to .south east of the node, similar for other corners (.north west etc.).
\draw [->,thick] (S.south east) --  node {\small{$63$}} (I) ;