Fitting lower battery boxes

Today I got the lower battery box back from rubberizing, that stuff sprayed onto pickup truck beds. It fits very snug around the right frame member. Raising it into place takes some effort, even empty. Harder still with 4 batteries loaded in the outer boxes, as then the weight tops 500 pounds.

Why install it with some of the batteries already inside? The rear interior side panels extend about 4" off the metal side wall. If I removed the decking under that, mud spray would be into that cavity.

I will cut up to the location of the interior sides. The opening will enable access to the battery terminals but will not be large enough to insert/remove batteries.

Thus, those batteries, 2 on each side, must be loaded in the box before it is raised.