How are input and output related? Maybe one of this methods do, what you need:
x = rand(51, 500);
y1 = x(:, 1:80); % First 80 columns
y2 = x(:, end - 79:end); % Last 80 columns
y3 = interp1(1:500, x.', linspace(1, 500, 80)).'; % Linear interpolation
