at work I have the task to convert a large library with Python 2.7 Code to Python 3.x.
This library contains a lot of scripts and extensions made with boost python for C
At the time of writing this, there are plans to support both Python 2.7 and 3.x in a single branch/version. Work on this feature has started, but it will take some more time to reach this goal.
So it looks as if your best bet would be to start right away. SCons itself should run fine under Python 2.7 for compiling the Boost extensions. The problem in your case are the added checks and detection mechanisms for deriving paths and module names from the version of the current Python interpreter. Since you can't give any more detail about this process, my answer is somewhat vague here, sorry. In principle you'd have to find the place in the SConstructs/SConscripts where the version of the currently running Python interpreter is determined. Just hardcode this to the 3.x version that you have installed on the machine additionally, and keep your fingers crossed that the rest will work automatically.
Note how there is a clear separation here between "compiling code for a Python version" vs "compiling code under a Python version".
In general, a better understanding of SCons internal workings and basic principles might be helpful. If you find the time, check out the UserGuide ( http://scons.org/doc/production/HTML/scons-user.html ) or consult our user mailing list ( see http://scons.org/lists.php ) for larger questions and discussions.