Cruising RPM. can it be to low ?

From a gas mileage point of view . Can cruising RPM be to low ? What would be a good RPM for getting the best mileage.? , for a mostly stock small block. Can the RPM be to low (under torque curve). With a set 2.76 or 3.23/ 27.4 dia tires / A500 with lockup/ .69to1 overdrive.
With the 2.76's I get a cruising RPM @60 of 1399.
With the 3.23's I get a cruising RPM@60 of 1641 (which I'm sure would be a better choice)
Would the lockup even be usable at 60 MPH at those RPM's ?