With that kind of torque, I guess lugging might NOT be an issue.
Depending on what your current gears are, Your equivalent final drive in 4th gear would be:
2.14 --> 1.43
2.29 --> 1.53
2.41 --> 1.61
2.56 --> 1.71
In effect, whatever your non-slip RPM is now (assuming you use lockup), it will be 2/3 of that after sticking in the overdrive.
I would not use an overdrive with those gears without the lockup, as that would cause a lot of trans fluid heating.
I think it could potentially hurt fuel economy, most especially with the 2.14 or 2.29. Maybe break even on the other two - the engine would just be revving way too low, and would be lugging (relatively speaking - you DO have a butt-load of torque).
In effect, if you were to get 3.23 gears, the overdrive would make that something like 2.16.
