I've not run nitro cars, so I don't know if you have the same issues. In an airplane, you get a several factors that change your mixture setting when you leave the ground.

The prop "unloads" and turns to a higher RPM than it does with the plane sitting still on the ground. It's a big effect.

The G forces frequently cause the engine to lean out a little. Espeically when you are climbing, nose up, the engine has to now draw fuel "uphill" against gravity. I don't think you get that situation in a car for as long as you do in a plane, though I could be wrong.

As the fuel level drops in the tank, the engine will lean out a little.

I will say this for sure, if you have your needle set a hair too lean, your engine will run fine sitting on the ground. But if you pick up the plane and point it striaght up, the RPM will drop a hair. If the needle is set right, you will not hear any RPM drop.

So, good practice is to run the engine to throttle, set the needle for a hair short of peak power, then point the airplane vertically. If you hear the RPM go down, richen 1-2 clicks and try again.

Some guys set the needle in the first place by pumping only a half tank, then holding the plane vertically, and peaking the RPM.

The nose down thing is to make sure you aren't too rich, that the engine won't flood and quit if the fuel system starts a syphon from the tank in to the carb in a dive.

Head pressure is basically the same reason people use water towers to get water in to your house. The higher the water level, the harder the water come out of the facuet, all powered by gravity. The same is true for a fuel tank, the fuel tries to run "downhill" into or out of the engine, changing the mixture slightly.

(btw, as a general habit, I try not to use the word "stall" in reference to the engine. The engine stops or quits or you go deadstick. The airplane "stalls" then falls out of the sky. That's just me though)