@Wittigstein...
Been a while but I thought you might be interested to know that the idea sort of worked :-)
One thing I overlooked was the FIR filter algorithm has a N-1 latency in that for a 63 tap filter, you get 63 less samples out than went in - bit of a problem for the window length.
Some small corrections in the overlapping sorted that out, but I dont think the FFT is too accurate on smallerwindows..
Maybe I'll take another look later, right now I dont have the time :-)