FFT - mixed radix - bit reversal
3 by 16 or 16 by 3 or 4 by 12 or 12 by 4 

Hope this helps, not the mixed case , but I think this is the right direction

I don't have a copy of 
Brigham, E. Oran, and E. Oran Brigham. The fast Fourier transform and its applications. Vol. 1. Englewood Cliffs, NJ: prentice Hall, 1988.

handy. I have a vague recollection that he had some mixed fft examples but I think was radix 4.  If you go to the fftw website and look at the papers section, there might be more help.

x=reshape(1:25',5,5)'  % read in row order fortran index
x =
     1     2     3     4     5
     6     7     8     9    10
    11    12    13    14    15
    16    17    18    19    20
    21    22    23    24    25
reshape(x,25,1)        % read out column order
ans =
     1
     6
    11
    16
    21
     2
     7
    12
    17
    22
     3
     8
    13
    18
    23
     4
     9
    14
    19
    24
     5
    10
    15
    20
    25

[y,i]=digitrevorder(0:24,5)  % c indexing 
y =
  Columns 1 through 20
     0     5    10    15    20     1     6    11    16    21     2     7    12    17    22     3     8    13    18    23
  Columns 21 through 25
     4     9    14    19    24
i =
     1
     6
    11
    16
    21
     2
     7
    12
    17
    22
     3
     8
    13
    18
    23
     4
     9
    14
    19
    24
     5
    10
    15
    20
    25
echo off