The generator matrix

 1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  2  1
 0  2  0  0  0  2  2  2 2a  0  2 2a 2a 2a 2a  2 2a 2a^2+2a+2  0  0  2 2a^2+2 2a^2+2a+2 2a+2 2a^2 2a+2  0 2a+2 2a^2 2a^2 2a^2+2a+2 2a^2+2a+2  2 2a^2+2a+2 2a+2  0 2a+2 2a^2+2 2a^2+2a+2  2  2 2a^2+2a
 0  0  2  0  0 2a^2+2 2a^2+2a+2 2a^2+2a 2a^2+2a  2 2a+2 2a^2+2  0  2 2a^2+2a+2 2a^2 2a 2a^2+2 2a^2 2a  0 2a^2+2a  2  0 2a^2+2a 2a^2 2a^2 2a^2+2 2a^2  0 2a^2+2 2a^2+2a+2  2 2a^2+2 2a^2+2a+2 2a^2+2a+2 2a^2+2a  0  0 2a 2a+2 2a
 0  0  0  2  0  2 2a^2+2a+2 2a 2a+2 2a^2 2a^2+2  0 2a^2+2a+2  2 2a 2a^2+2a 2a^2+2a 2a^2+2a  2 2a^2+2a 2a 2a 2a^2+2a 2a^2+2a 2a+2  0 2a^2  0 2a^2+2a  2  0  2 2a 2a+2 2a^2+2a+2  2 2a^2+2 2a+2 2a^2+2a 2a^2+2a+2 2a^2 2a
 0  0  0  0  2 2a^2+2 2a+2 2a 2a^2+2a+2 2a 2a^2 2a^2 2a+2  0 2a  0 2a^2 2a^2+2a 2a^2+2a  2 2a+2 2a^2+2a+2 2a^2+2 2a+2  2 2a 2a^2+2 2a^2  2 2a^2 2a^2+2a+2 2a 2a 2a^2+2 2a 2a^2+2a+2 2a^2+2 2a^2 2a^2 2a^2 2a 2a^2+2a

generates a code of length 42 over GR(64,4) who´s minimum homogenous weight is 256.

Homogenous weight enumerator: w(x)=1x^0+721x^256+1988x^264+2947x^272+3710x^280+28672x^287+4592x^288+200704x^295+5635x^296+5761x^304+4508x^312+2212x^320+623x^328+70x^336

The gray image is a code over GF(8) with n=336, k=6 and d=256.
This code was found by Heurico 1.16 in 24.9 seconds.