40mm = 1.57 inches, is pretty small for fitting a motor. Your best option for finding a motor would probably be a "right angle drive" type gearmotor. Still, it seems to me that turning the robot will be an issue. Having two drive wheels spaced only 4cm apart, attempting to rotate the robot of length 76cm (30 in) seems like it would require too much torque, and the wheels would not have sufficient traction. Most robots that use this "differential drive" type steering don't have a wheel-wheel distance vs length of robot ratio under roughly 1/2 but 4/76 is...a lot more.
I actually know of any gearmotors that are that small. Also, will the 2 motors have to be side by side? That would require each to be less than 0.8 inch in diameter. 6.6 lbs is also a lot for such small motors. Could you describe what terrain this robot will be in? Perhaps it would be easier to rethink the robot's method of locomotion rather than searching for motors that would fit these needs.