Cruising RPM. can it be to low ?

Before I installed the digital Megasquirt ignition on my 318, it would lug down in Overdrive and highway speeds and inclines. It had a 3.23 rear axle.
Changed to 3.91 gears which 'solved' this. Stepped down to 3.55 gears later on.
Then I installed a digital ignition and was able to tune the engine's ignition curve much better and I'm sure the car would now still be happy with 3.23 again.