The new cover definitely moves the oil level much higher. If you look at the before and after pics (linked above), the stock cover has the plug below the case's machining index holes. The new cover has the fill plug even with the top of those index holes, so it's almost 2" higher. I usually fill to the bottom of the fill hole. I think that puts the fluid level up to only 1/4-inch below the top of the axle tubes.
No issues with leaks or drips after several months of highway driving, including a few 100-mile trips with heavy loads. So there is still room for the fluid to expand.
But where the old level would have the axles barely dipping into the fluid (good enough to theoretically fling it around inside), the new level has the axles themselves pretty much immersed. Maybe B&M thinks that's needed for dragstrip use? I think it's overkill here.
Also, the stuff that leaked out was pretty gooed up with rust dust. Probably scrubbed off the insides of case and tubes. I used the Mobil 1 gear lube, and maybe it has that much better detergents.
No sign whatsoever of water intrusion. Yes, I've seen that before on my Dakota, and the D44 on the front of this rig was filled with an icky emulsion probably nearly half water. With hubs, it's not been run under load with that in there, and it hadn't been there long. I blame the PO's kid.
'Bottom of the fill hole' is an easy gauge. I guess I have to make up a 'dipstick' for checking the level. Certainly plan to drain this goop out and refill to a lower level. Good thing; the Mobil 1 stuff is expensive.