Well I am not sure it will affect the ammo box , because you are just removing a small amount of material from the back of the thigh. The plastic weld won't matter because you are not going to unglue the rear joining strip, just cut the plastic strip down along the edges, all the way thru the thigh and remove it.Then the back of the thigh piece would be open like it was when assembling it. Your thigh would be about an inch smaller in diameter. Then put on a new strip, on the thigh. When I assembled my forearms one of them was to way to big, And it was completely glued , so I thought about it for quite some time before getting the nerve to make it smaller. You wont be making it alot thinner,just enough to make them blend alittle better. If you dont feel comfortable doing this dont try it. Maybe someone near you with more experience could lend you a hand.
Edit: you look fine, but if you want to mod them, contact your local garrison for help