I'll post some pictures, yeah. Just so that there's no confusion, Metal Grudge does NOT have an entry on rebrickable, Girl's Tank does (and also his crazy looking snowmobile too)
I was considering filling the gap with a stack of plates the same size as the battery box, just to even out the weight distribution. However, I ended up filling some of that space with reinforcements for the motors (not really needed, but I just wanted to do it). With the lightweight rechargeable battery box there's no weight distribution problem at all. Tank goes straight when it's supposed to. I ended up adding a bit to mine as well because I didn't have all the original colours.
As for the 2 receivers vs. 1, his explanation makes sense, but the tank is very fast with just one receiver already, so I don't know if it would make sense to make it even faster. I'll give it a try - I ordered two receivers on his recommendation, but the thing already glides very fast, so I'd say it's not really needed.
I've just done wanted list on Bricklink for Girl's tank. It's... expensive... really expensive. $314 for the non-electrical parts (they are always cheapest from Lego directly). So probably $400 with all the electrical parts, as it needs 4 motors and 2 receivers. That's just way too much