I have turboed one of these engines, made a manifold, similar to the one above. MS is the best option, but for only a few psi you could retard the ignition a few degrees and use a fifth injector setup, one extra std injector. Wire it live with the rest of them and earth it via a prssure switch (set at 3psi) to one of the other injector earths, and so it will be pulsed on boost with the rest, giving you 25% more fuel on boost, a little rough but it works.