diff ratio, much of a difference?

To a certain extent the MPH tells the real power, the ET tells how well the car uses that power. The flatter the engine's power curves, the less effect a gear ratio change will have. You have a fairly stout engine there, but you will need a lot more info to choose the right gear for your car. That being said - in a general sense, assuming you can hook it, any drop in gear ratio (meaning higher numerically) will make it easier for that engine to accelerate the car. So any lower axle ratio should help the ET, that is until you go too far. 3.55 isn't anywhere near "too far".