2000 or lower is IMO, Not realistic from a fuel-economy standpoint, because there is no way to give the engine the timing that it wants, with the factory type distributor. So your combo is most likely to get LESS mpgs at 2000 than at 2400.
If you just want to cruise down there for other reasons, I cannot speak to that.
My combo now cruises at 65=2240 and She is about 6 to 8 degrees shy of getting the timing she wants. So I have installed a dash-mounted, dial-back, timing device, and crank it up come cruising time.