Just wanted to update this post, since it is hard to find information on mounting the trunk battery to the frame. This is how I ended up doing it, which i posted elsewhere and figured I'd share.
First I bolted a steel plate to the frame with 4" long bolts, after drilling holes through the 2 layers of sheet metal and the frame in the bolt locations. I put the holes in a spot where the bottoms of the bolts would be accessible through the body mount access holes on the frame. The bolts that go thru the frame and plate do not go thru the battery box, the box sits on top of it, this way i had a flat surface for the battery to sit on inside the box It's mounted through the top layer of the frame, though i assume you could go through both with a longer bolt.In any case then I bolted the battery box to the steel plate you see in the pic with the supplied hardware. I've added a lil CAD drawing. .I couldn't find any info of how ppl bolted it to the frame so this is what I came up with. I still have to add my cutoff switch.