My old 4 speed 440 Cuda with a Holly SD with a Holly 850 double pumper saw definite benefits from a 1/2 inch spacer, but no real difference between the 1/2 and 1 inch spacer. Keep in mind, that this was before dyno tuning was accesible, so the tuning was done seat of the pants and with the 1/4 mile timer. I am not sure how much spacer you can run with the SD before clearance becomes an issue.