Should I swap the 3.91's for 3.55 or 3.23

Of all the info you gave us, and that was great, you don't really say the "intended purpose." Are you more interested in loss at launch and low speed, or the effect at cruise, AND how fast you intending to "cruise?"

Here's a mph gear calculator from Wallace racing

http://www.wallaceracing.com/gear-speed.php