As much as I am NOT opposed to a five speed I have to say it really isn't necessary with a big block. Normally you need more gear ratios where you lack a broad torque curve. If I were looking at an overdrive I'd look at Passon's new 833-OD (which I believe can be had in an 18-spline). No extra mods, bolts in place just like the original and, I believe, considerably less expensive.