Good morning Irvin!
First off, the closer you get to finishing it, the more I realize that thing is a beast! That's gotta be just about the biggest patio-style offset smoker I've ever seen!
I agree with Keith
JckDanls 07
that you may have some heat damage of those rubber tires closest to the FB. Some heat-shielding fenders of some kind is a good idea.
And I see what you mean about balance - it does look like there's too much weight behind those rear wheels. Only 2 solutions come to mind, and it sounds like you've already thought about both:
1. Add weight to the front as a ballast. This is obviously a simpler and less time-consuming choice, but since this pit is already so massive, any extra weight is gonna make it more challenging to move.
2. Re-building the cart to make the wheelbase longer. This might not be an ideal solution either because, it looks like the only way to push the rear wheels further back is to raise the overall height of the pit. Might make it so high that it'd be less user-friendly, and would raise the center of gravity, perhaps creating new balance issues. I suppose you could both lengthen and widen the rear axle, pushing the rear tires back and out. That would obviously change the overall footprint - which might not be desirable.
Whatever you do, good luck! It's really looking good!
Red