I'm surprised you don't like the gearing. I had 3.21s (original-318/3-speed 73), and found the opposite: it was too much, 3000RPM was ~65mph.
I went from 3.21 to 2.76 with a 2.66 4-speed...and with my 360 Magnum, it's awesome. I wouldn't mind a 3.09 trans, but it's fine with what I have.