generating pilot sub-carriers in matlab
In your case, each OFDM symbol contains 256 sub-carriers. Store the pilot values for a OFDM symbol at time instant $n$ in an $8 \times 1$ array say V. Also, you have the positions of the pilot carriers stored in a array say, P. Let the OF(:,n) represent the $256 \times 1$ OFDM symbol at time n. 

Then you can assign the pilot values V to the corresponding pilot positions P in the OFDM symbol at time n as

OF(P,n)=V


in MATLAB. I hope this helps you a bit.