You should be able to just name the Python version you want. Tox will figure out the path.
basepython = python2.7
If that doesn't work for you, Tox supports environment substitutions inside curly braces.
{envname}
The name of the virtual environment
{envpython}
Path to the virtual Python interpreter
{envdir}
Directory of the virtualenv hierarchy
More info in the Tox documentation