get an OD transmission and forget about it. The 3.38 gears IMHO are a poor compromise. Keep the lower gears for their grunt and acceleration and swap transmissions to get an extra gear for less rpm, more economy and better highway speeds.
Assuming you have a 4.11 as you say (should be a 3.90) the 4.11 would be like a 3.0:1 in OD. The 3.90 would be about 2.85:1 final drive.
The typical OD transmissions will drop rpm by 27-30%
