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

generates a code of length 40 over GR(64,4) who´s minimum homogenous weight is 256.

Homogenous weight enumerator: w(x)=1x^0+343x^256+847x^264+714x^272+3584x^273+595x^280+25088x^281+567x^288+441x^296+364x^304+189x^312+35x^320

The gray image is a code over GF(8) with n=320, k=5 and d=256.
This code was found by Heurico 1.16 in 0.378 seconds.