Have Issues with Large Custom Tree

I've been having issues trying to generate these very large custom structure trees for a mod I'm designing. These trees fit in a 30x21x30 area, do not fit any vanilla options, and I have them checked to generate on grass blocks. The issues, though, are that the trees most the time generate in the air/over water/ inside ground/ inside each other/ etc. I think this is because the leaves of the tree are over grass blocks, but the stump itself isn't. 

My possible solution to this is to make it where it only checks if there's grass blocks directly under the stump of the tree. This and disabling the "generate only over grass blocks" this could solve my problem, I'm just unsure how I would make a procedure do that since I'm quite new to Mcreator (I installed it like a week ago). 

I've checked plenty of tutorials, but most of them didn't help. The trees do almost everything that I want them to do, or something that can be easily configured except this main issue. I feel like I'm almost reaching a solution but it's just outside my reach. If anyone could show me an image of a procedure that can do what my possible solution is, that would be greatly appreciated!